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 what if we rename Http2ToHttpHeaders->HttpUtil?  The name should be clear since it will be in the http2 package.  That way it could be more of a grab-bag of constants/methods for the HTTP conversion layer (including these).  WDYT?