feat: persistent storage for hub data

[varunsrin]

Hub data should be persisted to a reliable data store like Postgres, Redis or equivalent.

This is important for v2 because the Hub needs to be run as a service in the cloud that can be restarted. The correct long term design might be to use a low-level kv-store like rocksdb or levelsdb, but we're ok cutting corners a bit to ship quickly and then refactor it on later. It's important that we use a simple querying and table layout so that we can migrate later when necessary.

[pfletcherhill]

Rocksdb implemented with #107