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.
We weren't throwing anything before, so I don't think it's worth changing at the moment.  If we get frisky in the future we could take a pass through all of our collections and make sure we do this.  Sound reasonable?