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.
Are you sure we don't need a `childState.resetAllocatedPriorityBytes()` call here?  What if the first call to this method falls into here (and all subsequent recursive calls), where does the `allocatedPriorityBytes` get reset in this case?

Also if we do add a reset call here is there any chance for double reset (and does that matter) between this line and the existing reset call?