Changing HTTP/2 inbound flow control to use Http2FrameWriter

Motivation:

This is just some general cleanup to get rid of the FrameWriter inner
interface withing Http2InboundFlowController.  It's not necessary since
the flow controller can just use the Http2FrameWriter to send
WINDOW_UPDATE frames.

Modifications:

Updated DefaultHttp2InboundFlowController to use Http2FrameWriter.

Result:

The inbound flow control code is somewhat less smelly :).