This is part one of a small set of CL's that aim to resolve
the outstanding TODOs relating to channel close and blocking
behavior.

Firstly, the hairy handling of assigning the peersId is now
done in one place. The cost of this change is the slightly
paradoxical construction of the partially created clientChan.

Secondly, by creating clientChan.stdin/out/err when the channel
is opened, the creation of consumers like tcpchan and Session
is simplified; they just have to wire themselves up to the
relevant readers/writers.

R=agl, gustav.paul, rsc
CC=golang-dev
https://golang.org/cl/5448073