First implementation of minimal Broker API

I assume that most of the "heavy-lifting" i.e. pushing data around will be done PUB/SUB style. But in our earlier experiments, it turned out that setting up and destroying a network in a controlled way is something where you appreciate the blocking/synch behavior of REQ/REP. For example, a network management component (however that would look like) could as a service to create a PUB socket via REQ and then wait for it to ack the creation by blocking on the REP. Other synchronization patterns work just as well as use cases. 

It's not essential that we insert this right away, but again it's something to keep in mind for the not too distant future.