HTTP/2 Priority Algorithm Restructure

Motivation:
The current priority algorithm uses 2 different mechanisms to iterate the priority tree and send the results of the allocation.  The current algorithm also uses a two step phase where everything is traversed and allocation amounts are calculated and then everything is traversed again and everything that was allocated is sent.

Modifications:
- DefaultHttp2OutboundFlowController will allocate and send in the same looping structure.
- The recursive method will send only for the children instead of itself and its children which should simplify the recursion.

Result:
Hopefully simplified recursive algorithm with consistent tree iteration approach and 1 less iteration pass.
Hmm ... I think this may still result in > 1 write per stream.  Since this method is recursive, we could we calling it more than once on the same streams, each time with different connection windows.  Just because our assigned connection window is exhausted doesn't mean that we're completely done allocating to these streams.