Better context usage

Trying to minimize the number of ZMQ context a remus worker uses.

Previously the remus worker required an internal context for communication, but that is solved by using UUID's as the channel name.
Secondly we have reduced the number of zmq::context's that are created by ServerConnection in the default and copy constructor. 

The total result is that we have nearly tripled the number of remus Workers that can be inside the same process space.