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.
If we have a stream with a really high priority but almost no data, I'm changing the weight to be lower.  This will also mean that any streams with no data will have a new weight of zero.

Likewise, if we have a stream with a lot of data but a low priority, I'm bumping it up ... for throughput.