Fix bug in HTTP/2 outbound flow control

Motivation:

The outbound flow controller logic does not properly reset the allocated
bytes between successive invocations of the priority algorithm.

Modifications:

Updated the priority algorithm to reset the allocated bytes for each
stream.

Result:

Each call to the priority algorithm now starts with zero allocated bytes
for each stream.
Done ... returning the bytesWritten and logging it.  I'm no longer throwing Http2Exception ... it will never actually happen since we're decrementing anyway.  If it does, I now throw AssertionError.