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.
Actually the upperLimit variable should make no difference to that loop ... it's just a bit of cleanup since it didn't require 2 indices.

The decrease in number of streams is really just due to the increase in the payload size ... and to keep the test time to something reasonable.  We can probably bump this back up to something like 1000, but above that we may risk getting timeouts on slower machines.