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'm sure I'm missing something but would you mind clarifying the use of `totalAllocated` here?  Seems like `totalAllocated` gets incremented after every recursive call (on line 332).  Couldn't this potentially over-allocate?