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.
A few of my other comments got compressed related to this line I made else where.  Now I can make a comment on this line because there are changes near it.

Should this be `childState.resetAllocatedPriorityBytes()`?  The current call just results in a `allocatedPriorityBytes += 0` statement.