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.
I don't think any decision WRT twitter hpack interfaces has to slow us down here.  I don't think we would want our interface extending theirs anyway.

But sure, feel free to make a PR against their hpack.