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 just realized that we'll never operate on streams that can't be written to, so we don't need to worry about this.  We listen to the connection for stream events and clear all pending bytes if the stream is no longer writable.