Reducing memory usage for HTTP/2

Motivation:

The connection, priority tree, and inbound/outbound flow controllers
each maintain a separate map for stream information. This is wasteful
and complicates the design since as streams are added/removed, multiple
structures have to be updated.

Modifications:

- Merging the priority tree into Http2Connection. Then we can use
Http2Connection as the central stream repository.

- Adding observer pattern to Http2Connection so flow controllers can be
told when a new stream is created, closed, etc.

- Adding properties for inboundFlow/outboundFlow state to Http2Stream.
This allows the controller to access flow control state directly from
the stream without requiring additional structures.

- Separate out the StreamRemovalPolicy and created a "default"
implementation that runs periodic garbage collection.  This used to be
internal to the outbound flow controller, but I think it is more general
than that.

Result:

HTTP/2 classes will require less storage for new streams.