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.
Agreed it is a little confusing.  Since it's used in conjunction with `streamableBytesForTree`, maybe `allocatedBytesForTree` would be more appropriate?  I can comment the writing loop to make it clear how we're using this value during writing.  WDYT?