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.
Since `self._sensors` is being mutated by `_stop_sensor_process` method.

As an alternative (to avoid mutating dict during iteration), I could also do `sensor_ids = self._sensors.keys()`. This option is probably better and cleaner.