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.
Is is possible to remove this leading check and re-locate this logic so we don't have to have to keep track of `numPasses ` and have a special `if (numPasses == 0) {` run every iteration?   It seems like the allocated prioritized bytes is initialized to 0 in the class.  What about after the `bytesWritten += writeAllowedBytes(child, childState.allocatedPriorityBytes());` line?