Adding HTTP/2 application-level inbound flow control support.

Motivation:

Currently the DefaultHttp2InboundFlowController only supports the
ability to turn on and off "window maintenance" for a stream. This is
insufficient for true application-level flow control that may only want
to return a few bytes to flow control at a time.

Modifications:

Removing "window maintenance" interface from
DefaultHttp2InboundFlowController in favor of the new interface.

Created the Http2InboundFlowState interface which extends Http2FlowState
to add the ability to return bytes for a specific stream.

Changed the onDataRead method to return an integer number of bytes that
will be immediately returned to flow control, to support use cases that
want to opt-out of application-level inbound flow control.

Updated DefaultHttp2InboundFlowController to use 2 windows per stream.
The first, "window", is the actual flow control window that is
decremented as soon as data is received.  The second "processedWindow"
is a delayed view of "window" that is only decremented after the
application returns the processed bytes.  It is processedWindow that is
used when determining when to send a WINDOW_UPDATE to restore part of
the inbound flow control window for the stream/connection.

Result:

The HTTP/2 inbound flow control interfaces support application-level
flow control.
Consider moving this null check into `state(int streamId)`.  I think this is the only case where it is actually expected (and the return value is checked).