This refactors the PDS to use a single-tenant SQLite datastore.

Each user has their own SQLite file that stores their repo and private account state.
User databases are stored hierarchically as such: /${dbDirectory}/${sha256Hex(did).slice(0,2)}/${did}

Repo signing keys for each repo are stored alongside the SQLite file.

We also switch out the abstraction for interacting with user data to an ActorStore. The primary difference be