This CL introduces a new struct, channel to hold common shared
functions.
* add a new channel struct, which is embeded in {client,server}Chan.
* move common methods from {client,server}Chan into channel.
* remove unneeded used of serverConn.lock in serverChan
(transport.writePacket has its own mutex).
* remove filteredConn, introduce conn.
R=agl, gustav.paul
CC=golang-dev
https://golang.org/cl/6128059