Fixing startup race condition in HTTP/2 codec.

Motivation:

The Http2ConnectionHandler and Http2FrameEncoder both disallow writing
until the connection header and initial settings frame are sent. This
makes it difficult for downstream handlers to know when to begin sending
messages.

Modifications:

- Http2FrameEncoder: changed to no longer disallow writing frames from
downstream handlers

- Http2ConnectionHandler: Now sending initial settings frame upon
activation of the channel to make sure that it's sent before downstream
handlers have a chance to write additional frames.

Result:

Downstream handlers should be able to write frames as soon as the
connection is active.