Improve thread safety in HeartbeatChecker and heartbeat updates in Controller

this MR changes in HeartbeatChecker

• add Lock to make access to zmq.Poller and zmq.Socket (more) thread safe
• allow to unregister Satellites
• rename logger to make it consistent with the rest of the package: constellation.core.heartbeatchecker

This allows to improve clean-up of heartbeat connections in the Controller: previously, stale connections would remain even after a Satellite has been restarted; this MR fixes that.