Refactorize HeatbeatChecker, fixes for caenhv

This MR refactorizes the HeartbeatChecker to use zmq.Poller to improve the performance (which, in turn, makes the Controller much more responsive with many active Satellites: on our DAQ system with 26 Satellites, the Controller used to idle at 60% of a CPU core; now it is less than 1%).

Furthermore, various fixes for the CAEN HV Satellite are included, a fix for the shutdown command, refined logging messages and some refactorized tests. The latter were necessary, as I seem to run into every possible race condition and sometimes-but-not-always problem that the python test were exhibiting with this MR. I have hopefully fixed the main offenders now.