Cleaning up HTTP/2 outbound flow control

Motivation:

The outbound flow controller currently has to walk the entire tree each
time to calculate the total available data for each subtree. For better
performance we should maintain a running total for each subtree as we
queue/write frames.

Modifications:

I've modified the DefaultHttp2OutboundFlowController to manage the state
of "priorityData" at each node in the priority tree, which is
essentially the total writable data for all streams in that subtree.
These totals do not take into account the connection window, as that is
applied when splitting the data across the streams at each level in the
tree.

The flow controller now sorts the children of a node by the product of
their data (for the subtree) and its weight. This is used since in
certain cases the algorithm might prefer nodes that appear later in the
list. Sorting helps keep nodes with similar characteristics (e.g. a lot
of data and a high priority) with similar output.

To help clean things up, I'm storing a FlowState for the root node as
well, which maintains the runnning total of the currently writable data
for all stream.

Another item of cleanup is that I created a GarbageCollector innerclass
within the outbound flow controller. This keeps all of the garbage
collection code in one place, away from the flow control code.

Also added some more unit tests for the flow controller.

Result:

The outbound flow controller is a bit cleaner and perhaps a bit more
fair when distributing outbound data across streams.