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 clarify when we would want to allow sending the `extra` amount?  Seems like `allocateBytes` should be returning the maximum amount of bytes allowed at the current subtree (upper bounded by the `connectionWindow` parameter).