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.
What are your thoughts about changing the Http2Stream.children interface to return a `List<...>`?  We would lose some "genericness" but it would avoid another copy operation.  I think the current uses of the method permit this.