Improve the throughput of Actors using ZeroMQ and the Request/Reply pattern.

Following this discussion https://groups.google.com/forum/#!topic/akka-user/lGBb8GTouV0

I propose a new implementation of the ConcurrentSocketActor in akka-zeromq.

This new version does not rely on polling at all and has faster throughput.

I also made a sample application (akka-sample-zeromq-reqrep) for testing purpose.