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.
Hmm ... the name "handler" is slightly misleading since in Netty it is typically used to represent something implementing `ChannelHandler`.  I wonder if there might be a benefit to making this extend `ChannelHandlerAdapter` ... perhaps we'll eventually want to handle generic channel writes for something?  Perhaps not :) ... any thoughts?