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.
Should we be checking if the stream is in an [appropriate state](https://tools.ietf.org/html/draft-ietf-httpbis-http2-15#section-5.3.4) (i.e. not closed?) before allocating to them?