Enable ability to create named in memory stores and in memory stores with shared caches.
Include ability to create encrypted connections.
Update `Store` to take an `Arc<Conn>` so references can be shared.
Update FFI to use `Stores` instead of `Store`.
Add `store_open_named_in_memory_store` to open a named in-memory store over FFI (useful for tests).
Sync needs to operate over a "mentat transaction", not just a "db transaction".
This shuffle allows internal mentat crates to consume InProgress, which models
the concept of a "mentat transaction".
