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.
Sounds reasonable.  Is this something `allocateBytesForTree` could take care of on the way up from recursion?  Seems a bit awkward to have `allocateBytesForTree` get all the children and do iteration one way (via tree methods) and then have an external method do an iteration another way (all active nodes).