Review Device-Host mapping

[arkocal]

This is a long term issue.

We need to review how we map devices to websocket server instances, an we will have to do the same for mqtt. As of now, we are creating an entry in Postgres and frontend looks this value up.

Problems:

• This means lots of writes to Postgres after setup, which is not good for horizontal scalability. We should consider moving this to redis
• We are not sure whether all state transtions are handled correctly (connection closed by client/ websocket server goes down, postgres goes down etc.) We should write more tests and implement the missing functionality.
• This is implemented as a Deployment in Kubernetes, but it should be a stateful set instead.