Global Changes:
  * The plugin was using duplicate definitions of internal DAOS client
    structures (dfs_obj_t, dfs_t, dfs_entry_t), and would create malloc'd
    copies of those structs in order to be able to access their private
    fields.  Should DAOS modify those structures in future releases, the
    plugin would break for those releases.  The dependencies on internal
    fields have been removed, the DAOS client API is now strictly followed.
  * The path_map and size_map caches used DFS mount-point-relative
    pathnames as keys.  If more than one DFS filesystem is mounted during
    the same run, then the same relative pathname could be in use in
    both filesystems.  Callers that retrieved values from the caches
    could get results for the wrong filesytem.  Code was changed to create
    path_map and size_map caches per-filesystem.
  * A number of fields (connected, daos_fs, pool, container) were
    stored in the global DFS structure which meant that any time a
    path was presented to the plugin that was in a different DFS
    filesystem than the previous path, the current filesystem would
    have to be unmounted and then the new filesystem would be mounted.
    The application could have had files open at the time in the
    filesystem that was unmounted.  The code was changed to maintain
    filesystem state relative to each pool/container combo, and so
    any number of DFS filesystems can now be mounted simultaneously.
  * None of the code in the DFS Cleanup() function was ever being called.
    This is a known tensorflow issue, see
        tensorflow/tensorflow#27535
    The workaround is to call Cleanup() via the atexit() function.
  * The RenameFile function was enhanced to delete the cached size
    of the source file and store that cached size for the destination
    file.
  * The dfsLookUp() routine required the caller to indicate whether
    or not the object to be looked up was a directory.  This was
    necessary because dfs_open() was being used to open the object,
    and that call requires a different open_mode for directories and
    files.  However, the caller does not always know the type of the
    object being looked up, e.g PathExists() and IsDir().  If the
    caller guesses wrong, then the dfs_open() call fails, either with
    EINVAL or ENOTDIR.  The caller would map these errors to ENOENT,
    which is incorrect.  Code was changed to replace the dfs_open()
    call with dfs_lookup_rel(), which removes the requirement that
    the caller know the object's type a priori, the caller can check
    the type of the object after it has been opened.
  * The dfsLookUp() routine required all callers to implement three
    different behaviors depending upon the type of object being opened.
    1. If a DFS root directory, a null dfs_obj_t would be returned,
       this would have to be special-cased by the caller.
    2. If a non-root directory, a non-null dfs_obj_t would be returned
       which the caller must never release because the dfs_obj_t is
       also an entry in the path_map cache.  Releasing the entry would
       cause future requests that use that cache entry to fail.  There
       were a few cases in the code where this was occurring.
    3. If a non-directory, a non-null dfs_obj_t would be returned
       which the caller must always release when done with it.
    The code was changed so that a DFS root directory returns a
    non-null dfs_obj_t.  Also, whenever a directory that is in the
    path_map cache is referenced, dfs_dup() is used to make a (cheap)
    copy of the dfs_obj_t to return to the caller, so that the cached
    copy is never used outside of the cache.  As a result, dfsLookUp()
    now always returns a non-null dfs_obj_t which must be released when
    no longer in use.  Another advantage of using dfs_dup() is that it
    is then safe at any moment to clear a filesystem's path_map cache,
    there is no possibility that some caller is using a cached dfs_obj_t
    at that time.
  * All relative path references in the code have been replaced with
    references to a dfs_path_t class which encapsulates everything
    known about a particular DFS path, including the filesystem in
    which the path resides.  Member functions make it easy to update
    the correct caches for the correct filesystem for each path.
    Also, there were many places throughout the code where string
    manipulation was being done, e.g. to extract a parent pathname or
    a basename.  That code has been replaced with dfs_path_t member
    functions so that the actual string manipulation only occurs in
    a single place in the plugin.
  * Setup() now initializes a dfs_path_t instead of global pool, cont,
    and rel_path variables.  It also does some minor lexical
    normalization of the rel_path member, as opposed to doing so in
    multiple places in the code downstream.
  * Code was modified in various places so that 100 of the tests in
    the tensorflow modular_filesystem_test' test suite pass.  there are
    three remaining failing tests.  One is an incorrect test, one is
    checking a function not implemented in the plugin.  The third is
    reporting failures in TranslateName() which will be handled in a
    separate PR.
  * The plugin was coded to use 'dfs://' as the filesystem prefix, but
    the DAOS client API is coded internally to use 'daos://' as the
    prefix.  The plugin was changed to use 'daos://' so that pathnames
    used by one application would not have to be munged in order to
    also work with tensorflow.

Per file changes:

dfs_utils.h:
    * The per-container class cont_info_t was added that maintains all
      per-filesystem state.
    * Class dfs_path_t was added that maintains all per-file state.
      The class knows which filesystem the file is in, e.g. to update
      the correct cache maps.
    * The global fields connected, daos_fs, pool, container, path_map,
      and size_map are removed, replaced by the per-filesystem versions.
    * Mounts are now done at the same time as connection to the container,
      filesytems remain mounted until their containers are disconnected.

dfs_filesystem.cc:
  * Many of the functions were made static so that they don't show up
    in the library's symbol table, avoiding potential conflicts with
    other plugins.
  * Changed path references to dfs_path_t references throughout.
  DFSRandomAccessFile()
    * Replaced the dpath string with the dfs_path_t as a constructor
      parameter so that the per-filesystem size cache can be updated.
  DFSWritableFile()
    * Replaced the dpath string with the dfs_path_t as a constructor
      parameter so that the per-filesystem size cache can be updated
      whenever the file is appended to.
  NewWritableFile()
    * Changed file creation mode parameter to include S_IRUSR so that
      files can be read when the filesystem is mounted via fuse.
  NewAppendableFile()
    * Changed file creation mode parameter to include S_IRUSR so that
      files can be read when the filesystem is mounted via fuse.
  PathExists()
    * Reworked the code to work with the new dfsLookUp() behavior.
      dfsPathExists() call was removed as it no longer provided anything
      not already provided by dfsLookUp().  Also, many errors returned
      by dfsPathExists() were mapped to TF_NOT_FOUND, which was
      incorrect.  Also, PathExists() can be called for either files or
      directories, but dfsPathExists internally called dfsLookUp() with
      isDirectory = false, so callers that passed in a directory path
      would get failures.
  Stat()
    * Used to call dfs_get_size(), then called dfs_ostat(), but the file
      size is available  in stbuf, so the dfs_get_size() call was
      extra overhead and was removed.
  FlushCaches()
    * Used to call ClearConnections, which unmounted any filesystem and
      disconnected from its container and pool, when there could be
      files open for read or write.  The ClearConnections call was
      removed.  Code was added to clear the size caches as well as the
      directory caches.

dfs_utils.cc
  * New functions were added for clearing individual filesystem caches
    and all filesystem caches for all mounted filesystems.
  * There was code in many places for path string manipulation, checking
    if an object was a directory, etc.  dfs_path_t member functions were
    created to replace all these so that a particular operation was only
    implemented in one spot in the code.
  DFS::~DFS()
    * The code to clear the directory cache only released the first entry,
      there was no code to iterate through the container.  Replaced with
      function calls to clear all directory and size caches.
  Unmount()
    * Now done automatically as part of disconnecting a container, a
      separate function was no longer needed.
  ParseDFSPath()
    * The code assumed that any path it was given would have both pool
      and container components, it was unable to handle malformed paths.
      Code was changed to let duns_resolve_path() validate the path
      components.  There used to be two calls to duns_resolve_path()
      because DUNS_NO_CHECK_PATH was not set, and so the first path
      would fail if the pool and container components were labels,
      duns_resolv_path() only recognizes uuids if DUNS_NO_CHECK_PATH
      is not set.  When pool and/or container labels were used, the
      duns_resolve_path() code would check against local mounted
      filesystems, and would hopefully fail.  The code then prepended
      dfs:: and tried again, which would be recognized as a
      "direct path".  Paths which only contained uuids were
      successfully parsed with the first duns_resolve_path() call.
      By using the DUNS_NO_CHECK_PATH flag and always including the
      daos:// prefix, only a single system call is needed.
  Setup()
    * Reworked to populate a dfs_path_t instead of separate pool,
      cont, and relpath variables.  A filesystem is now automatically
      mounted as part of connecting to the container, so a separate
      function was no longer needed.
  ClearConnections()
    * The code for looping through pools and containers didn't work
      properly because the subroutines erase their map entries
      internally which invalidates the iterators being used in
      ClearConnections().  Code was changed so that the iterators
      are reinitialized each time through the loop.
    * Code to disconnect all the containers in a pool was moved to
      the DisconnectPool() function, so that it is not possible to
      disconnect a pool without first disconnecting all its containers.
  dfsDeleteObject()
    * Enhanced to only clear the directory cache for the filesystem in
      which the object existed.
    * If the object was a file, the size cache entry for that file is
      deleted.  If a directory was being recursively deleted, the
      filesystem's size cache is now also cleared.
  dfsLookUp() and dfsFindParent()
    * As mentioned at the top, code was rewritten so that cached
      directory entries are never returned to a caller, instead
      a dup reference is returned so that the caller is always
      given an object reference it must release.
   dfsCreateDir()
    * Error exit statuses were enhanced in order to pass the tensorflow
      'modular_filesystem_test' test suite.
  ConnectPool()
    * Simplified somewhat as the pool id_handle_t is no longer needed.
  ConnectContainer()
    * Simplified somewhat as the cont id_handle_t is no longer needed.
    * Added code to immediately mount any container that is connected.
    * Code added to initialize all the per-filesystem state variables
      was added.
  DisconnectPool()
    * Added code to disconnect any containers before disconnecting the
      pool.
  DisconnectContainer()
    * Added code to unmount any filesystem before disconnecting its
      container.
  * Added all the dfs_path_t member function implementations.
  * Included a few new dsym references for dfs function calls that have
    been added.

Signed-off-by: Kevan Rehm <kevan.rehm@hpe.com>