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.
1) The connectionWindow is changing on every recursive call, so this wouldn't work.
2) we could do this, but we'd still have to account for streams that still have data.
3) Again, the connectionWindow is relative to this particular invocation.  We may be called again with a different connectionWindow.

Not sure it's worth it to try to figure out an optimization for this ... it's pretty fast.