Call ctx.flush() at least once in ChunkedWriteHandler.flush()

Related: #3219

Motivation:

ChunkedWriteHandler.flush() does not call ctx.flush() when channel is
not writable. This can be a problem when other handler / non-Netty
thread writes messages simultaneously, because
ChunkedWriteHandler.flush() might have no chance to observe
channel.isWritable() returns true and thus the channel is never flushed.

Modifications:

- Ensure that ChunkedWriteHandler.flush() calls ctx.flush() at least
  once.

Result:

A stall connection issue, that occurs when certain combination of
handlers exist in a pipeline, has been fixed. (e.g. SslHandler and
ChunkedWriteHandler)
HTTP/2 Draft 16

Motivation:
HTTP/2 draft 16 has been released https://tools.ietf.org/html/draft-ietf-httpbis-http2-16.

Modifications:
The HTTP/2 codec should be updated to support draft 16.

Result:
HTTP/2 codec is draft 16 compliant.