persistent MQTT messages

We need to have persistent client sessions with QoS>0 to prevent data loss due to temporarily not available services.

• MQTT-Broker needs to be set to allow persistence
  • add ENVVARs MQTT_PERSISTENCE=true (and possibly MQTT_PERSISTENCE_LOCATION, MQTT_PERSISTENCE_FILE if they're not default values)
  • or configure mosquitto.conf
• define QoS>0 where needed (workers, frontend, object-storage, ...)
  • see issue tsm-dispatcher#29 (closed)