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.
`allocateBytesForTree` is iterating based upon tree traversal and tree links.  `writePendingBytes` is iterating based upon `connection.activeStreams()`.  These are the disjoint interfaces I was talking about.