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 not sure I totally understand the concern.  It's just a 2 step process: 1) allocate bytes 2) write bytes.  Can you be more specific about what you mean by disjoint interfaces?