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.
Can you explain this scaling?  I think it is either a 1 or 0.  Seem like the 0 will mean there will be no bytes written even when we may write some?  We may be dropping precision a bit to early?

Also is there a reason why this structure is advantageous as apposed to restructuring the algorithm to do BFS?  Seems like we are looping/scaling twice this way?