HTTP/2 to HTTP priority translation

Motivation:

The priority information reported by the HTTP/2 to HTTP tranlsation layer is not correct in all situations.
The HTTP translation layer is not using the Http2Connection.Listener interface to track tree restructures.
This incorrect information is being sent up to clients and is misleading.

Modifications:
-Restructure InboundHttp2ToHttpAdapter to allow a default data/header mode
-Extend this interface to provide an optional priority translation layer

Result:
-Priority information being correctly reported in HTTP/2 to HTTP translation layer
-Cleaner code with seperation of concerns (optional priority conversion).
@Scottmitch I think the reason was an needless optimization.  The idea is that priority is only used for outbound flow control and if we can't write (i.e. half closed local) there is no need to update the priority tree.  I think removing that if would help to simplify the logic though .... want to make that change in this PR?