Add a "multiplex" BackingStore driver

More than likely, to be useful, this thing needs to be able to handle multiple backing stores (Database, redis, and external services like Stripe). Currently, one *could* do this using multiple cloverleaf stores, but that seems inelegant and bug-prone.

Since the whole idea of this is to reduce the chances of bugs in your server, it would probably help to be able to handle all of these in a single `Store` instance. That way a single state update can handle updating the app database, the cache, and any third-party services in a way that keeps them consistent.

The issue does not name a file, test, or entry point. Start by locating the Store and BackingStore implementations, then determine how one store could coordinate database, Redis, and external-service updates. Done would require an agreed design and verified consistency across the requested backing stores.

Written by the indexing model from the issue text.