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
limitations:
poor error handling
teardown not done
exporter must send, importer must receive
testing is rudimentary at best
R=rsc
CC=golang-dev
https://golang.org/cl/186234
