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 see now my response to your comment doesn't make much sense...I thought you had made a comment later on suggesting to remove this interface but I think you just proposed this now ;)

I agree that keeping it symmetric is a good idea.  I will also make similar changes to `Http2OutboundConnectionHandler`.