netchan: use acknowledgements on export send.

netchan: use acknowledgements on export send.
Also add exporter.Drain() to wait for completion.
This makes it possible for an Exporter to fire off a message
and wait (by calling Drain) for the message to be received,
even if a client has yet to call to retrieve it.

Once this design is settled, I'll do the same for import send.

Testing strategies welcome.  I have some working stand-alone
tests.

R=rsc
CC=golang-dev
https://golang.org/cl/2137041