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.
Kind of renaming may help but it is a bit confusing to be combining tree and non-tree quantities. (i.e. line 284 `int streamable = Math.max(0, state.streamableBytes() - state.allocated());`).