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.
What exactly are you suggesting that `allocateBytesForTree` do?  Writing?  I don't think it can do the write, since we may end up calling allocateBytesForTree on the same children again in the future due to iteration.  If we only want to write once per stream it's best done separately, after all the recursion has occurred.