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.
Ok. Just to give some background...I was wrestling with the symmetry between `Http*Reader` classes being for lower level frame events that do work to feed the `Http*Listener` classes.  I was trying to accomplish the same relationship with `Http*Writer` and `Http*Producer` classes.  The writer is for lower level actual writing and the producer is a higher level concept that is not actually doing any writing.