Enable switchable backends for storage

[shreyashankar]

Redis will only take us so far, when sometimes it can be impractical to store everything in-memory. We want to support multiple backends---at least Redis and RocksDB for now, and if it becomes clearer to integrate another KV store-like DB, or if we need a relational model, we can consider other DBMSes.

[shreyashankar]

We can also have a Redis layer + RocksDB layer

[shreyashankar]

Can also consider s3 & filesystem

[shreyashankar]

We should have a storage interface, such that any storage system can be used for the backend.