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.
I think this is ok.  The `allocatePriorityBytes` method is only called from within the priority loop.  Within the loop, before each stream is completed it must pass through the `else` block on the inner loop which calls `childState.resetAllocatedPriorityBytes()`.