Rework zmq setup and cleanup

Register config options at import time and remove the register_opts()
function. It seems this isn't being used.

Add accessor methods for ZMQ_CTXT and matchmaker so that they are
lazily created rather than created at import time.

Remove the rpc_zmq_matchmaker override in the tests since
MatchMakerLocalhost is the default anyway. We were setting it with
an incorrect value, but only after the matchmaker had already been
created.

Change-Id: I2520252c64d70e4c4903e34d07952fed43e70ebe
Merge "Rework zmq setup and cleanup"