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.
'writePendingBytes` is what calls `allocateBytesForTree` ... it's the guy that runs the show.  It allocates first then writes.  I don't think this is so bad ... do you have a suggestion for how this might be cleaned up?