Cleaning up code for determining whether the HTTP/2 connection header
and initial settings frame were sent.

Motivation:

The current logic for marking the connection header and initial settings
frame is a bit complicated.

Modifications:

- Http2FrameEncoder, Http2ConnectionHandler: removed sent listeners and
set the sent flag immediately.

Result:

Just a code cleanup ... behavior is the same.
Merge branch 'http2' of https://github.com/nmittler/netty into http2