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.
@Scottmitch I think the simplest way to fix this is to get rid of this altogether and just make one last pass in writePendingBytes.  It's only O(n) and the cost of writing the frames and flushing will far outweigh the iteration.

WDYT?