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 see the utility here but I'm wondering if this should be called on line 388 instead (or also)?  Currently `nextState.allocatePriorityBytes(0);` is called which results in a `allocatedPriorityBytes += 0;` statement.