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 between the ActorStore and our previous "services" is that the ActorStore has different classes for reading & writing. Since SQLite does not support concurrent transactions, the ActorStore requires that you clearly "transact" with the store in order to make a write.

We also maintain an LRUCache for signing keys and databases. We allow 30k open file handles and 30k keys held in memory. When a database falls out of the cache, we ensure that we close the file handle.

We also introduce 3 separate SQLite databases for managing service state:

• service DB for account info, invite codes, refresh tokens etc
• did cache DB with only one table for caching DID resolutions
• sequencer DB with only one table for sequencing repo updates across all repos on a service.

Each of these SQLite files in run in WAL mode to allow for concurrent reads and streaming replication.

We intend to ship the PDS distribution with Litestream or something similar