HTTP/2 Codec Read/Write Restructure

Motivation:
The HTTP/2 codec has some duplication and the read/write interfaces are not cleanly exposed to users of the codec.

Modifications:
-Restructure the AbstractHttp2ConnectionHandler class to be able to extend write behavior before the outbound flow control gets the data
-Add Http2InboundConnectionHandler and Http2OutboundConnectionHandler interfaces and restructure external codec interface around these concepts

Result:
HTTP/2 codec provides a cleaner external interface which is easy to extend for read/write events.
We could put any common code into Http2CodecUtil if it's useful and makes sense.  My main concern is that we're getting a bit of interface bloat here ... and starting to run out of reasonable names as a consequence.  I'm fine with a bit of duplication so long as any real logic isn't duplicated (e.g. put it in a shared util class).