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 follow the benefits.  Is this inline with https://tools.ietf.org/html/draft-ietf-httpbis-http2-15#section-5.3.2?  Seems like we should not be manipulating the priority but instead allocating a percentage of the bytes to all the streams until we run out?