When data is received for a channel, but that channel
is not ready to receive it, the central run() loop
is currently blocked, but this can lead to deadlock
and interference between independent channels.
This CL adds an explicit buffer size to netchan
channels (an API change) - the sender will not
send values until the buffer is non empty.

The protocol changes to send ids rather than channel names
because acks can still be sent after a channel is hung up,
we we need an identifier that can be ignored.

R=r, rsc
CC=golang-dev
https://golang.org/cl/2447042