Fix a bug with child processes not being killed when stopping a sensor container

This fixes a bug with child processes not being killed when stopping a sensor container.

Eventually, we should add support for "graceful" shutdown to all the services (e.g. send `SIGUSR1` to the process first, wait x seconds and if process is not shut down yet, send `SIGTERM` then). This means draining the connections in the API server, invoking cleanup for all the sensors in the sensor container, etc.
+1 to .keys() approach. Copy seems like an overkill IMO.