Shared subscription in MQTT

[oscgonfer]

Check here: emqx/emqx#13566

TODO:

• Two new "roles" of MQTT subscriber. "main" and "secondary", "main" subscribers have a persistent session, and "secondary" ones do not.
• Update docker compose to allow horizontal scaling of both mqtt-tasks, and to provide a unique, but persistent client name somehow to MAIN subscribers. (is there a number in an an environment var or the hostname?). We need at least two "main" subscribers (as if both go down, we start losing messages). Secondaries can be added to and removed on demand.
• Add Docker-compose healthchecks and restart policy for mqtt-task (how?)
• Update mqtt task to pass a persistent client_id when subscribing, and to subscribe to to a shared subscription topic with THE SAME group eg $share/group/the/old/topic , setting clean_session to false on main subscribers, and to true on secondary.
• Ensure that Main subscribers are ALWAYS bought up BEFORE secondaries, and shut down AFTER, in docker (how)? (tested, unnecessary)
• Test extremely thoroughly on staging
• Ptest should send N messages (where n is number of MAIN subscribers) and checks that they are ALL received