=zer #3512 Harden zeromq ConcurrentSocketActorSpec

* 3 sec timeout waiting for terminated increased to 5 sec,
  ConcurrentSocketActor postStop (poller.unregister or socket.close)
  might perhaps take a while
* message from previous test step interleaved with next step, use
  separate endpoints for each step
Merge pull request #1836 from akka/wip-3512-zeromq-test-patriknw

=zer #3512 Harden zeromq ConcurrentSocketActorSpec