Pzq cluster feature with replications

Two main additions:
- the first one was to make pzq comptaible zith zeromq 3.x
- the second and most important one is adding the concept of cluster and replications in pzq. The nodes communicate in order to ensure high availability of messages that were accepted by pzq.

Of course it should continue to work without replication with at no cost.

It probably still needs some improvements but it should already be usable.