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'm not sure how that would avoid a copy ... `addAll()` is still a copy.  The call to `stream.children()` doesn't copy, it just wraps the list so that it's unmodifiable. Also, the constructor to `ArrayList` will take care of the sizing.  I don't see how passing in the collection would be an improvement.