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.
True.  Just a shame to have to copy internally to the codec...

What about providing an interface like `void children(Collection<? extends Http2Stream> collection);` in `Http2Stream` which will do a `collection.addAll(...)`.  This way they passed in collection could be sized before hand via `numChildren()` and avoid a copy?