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 to be honest I don't really have a strong preference one way or another, just wanted to bring it up as a point of discussion. :)

WRT having 2 classes, I think we have 2 options (with/without priority) regardless.  With my suggestion, rather than passing in a parameter to the constructor (translatePriorityEvents) you would just pick which version of InboundHttp2ToHttpAdaptor you wanted (e.g. the default or PriorityHttp2ToHttpAdaptor).