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.
Ah right, that's a little confusing ... should probably rename some variables and add some comments to make it more clear.  So the allocation is really for the entire tree.  If we allocate data to a stream A and its children B and C, A will be over-allocated because it will contain the total allocation of A+B+C.  That's ok, since when we go to write A, we'll only write the actual data available for A.  But for the algorithm to work, the allocated data needs to be for the entire tree ... make sense?