This laids the groundwork for sending messages through ChannelBuffers with channel-specific callbacks rather than a single onPlatformMessage callback.

This allows us to remove the logic from the framework that puts data back into the channel buffers. Right now (before this PR) the logic for messages from plugins to the framework is bidirectional:

```
                  **                 *
Plugins -> Engine -> ChannelBuffers <- Framework <---+-.
               |              |                      | |
               |              '------> via drain ----' |
               |                                       |
               '----------------- onPlatformMessage ---'

* = when the listener is null on the framework side
** = when onPlatformMessage is null
```

This ends up with weird race conditions and is generally less than completely clear. With this PR, we lay the groundwork for eventually reaching this model:

```
Plugins -> Engine -> ChannelBuffers -> Framework
```

...which is significantly simpler.