fs/p9p/new: refactor channel framing After a few connection bugs and framing issues, the transport has been refactored into a channel that manages raw message sending. More importantly, this frees up pre-session protocol version negotiation. To support this, the encoder and decoder have been folded into a Codec interface. The encoding support still needs work, but this has made the server and client code much simpler to understand. Signed-off-by: Stephen J Day <stephen.day@docker.com>