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.
Valid concerns. I'll remove the common interface/implementation and duplicate the code and we can take a look.

The class is simple so it should be fine but the concern was the `maxFrameSize`.  I thought I remember some issues with the exceptions or bounds checking being consistent before when code was duplicated.