[FEATURE] Use persistent sessions

[perbu]

Metamorphosis currently works on a best effort basis. In order to minimize the chance of losing messages I'm thinking about doing the following:

• Enable persistent MQTT sessions. This will ask the MQTT broker to retain messages if Metamorphosis is offline.
• Only acknowledge MQTT messages once we've written them to kafka

Once these are in place we can a bit more aggressive when we shut down, which should shorten the window we're down during a pod restart.