DM-1138: Store and retrieve last configuration - proof of concept

Created by: mattclarke

On a command message it will store a complete list of everything being forwarded in a Kafka topic (as JSON).

On restart it will retrieve the last set of settings and start them forwarding.

This is all optional (enabled by defining the storage topic) and the retrieval on start-up can be skipped using a flag.

Black and mypy were fighting on my mac, so I'll need to see what Jenkins thinks