Refactoring outbound flow controller to use frame writer.

Motivation:

This is addressing a TODO in the outbound flow controller. We currently
have a separate writer interface passed into the outbound flow
controller. This is confusing and limiting as to how the flow controller
can perform its writes (e.g. no control over flushing). Instead it would
be better to just let the flow controller use the Http2FrameWriter
directly.

Modifications:

- Added a new Http2DataWriter interface, which is extended by
Http2FrameWriter and Http2OutboundFlowController.

- Removed automatic flushing from Http2DataWriter in order to facilitate
optimizing the case where there are multiple writes.

- Updated DefaultHttp2OutboundFlowController to properly optimize
flushing of the ChannelHandlerContext when multiple writes occur.

Result:

Code is greatly simplified WRT outbound flow control and flushes are
optimized for flow-controlled DATA frames.
Maybe we should have the `ChannelPromise` as last argument in all methods to be more in line with `ChannelHandler`, `Channel`etc.