HTTP/2 Write Compression Support

Motivation:
The HTTP/2 codec does not supply an interface which facilitates doing data write compression.
Compressing data is anticipated to be a common use case and this feature is thus anticipated to be useful.

Modifications:
-Restructure the AbstractHttp2ConnectionHandler class to be able to extend write behavior before the outbound flow control gets the data
-Add classes which override the newly restructured interfaces to provide an extensible compression interface
-Add unit tests to test the new capability

Result:
HTTP/2 codec provides an interface (and default implementation) for data write compression.
@nmittler - This is where things come together...Is is a bit of a mash together of interfaces with some helper methods to allow what was the `AbstractHttp2ConnectionHandler` to accomplish what it needed.  I think this would be the place that may benefit from some re-arranging, clean up, or possibly a better name?  WDYT?