Move leap.common.events to use zmq

And ditch completely protobuf-zmq. #5718 (closed) is specific for the client/backend needed changes, but we probably can just use the same leap.common.events api and have the backend creating the sockets for us.

(from redmine: created on 2014-11-12, closed on 2015-06-01, relates #5718 (closed), relates #6360 (closed), relates #7127 (closed))