Right now you can't actually run spacetimedb-standalone in memory even though we have an --in-memory flag.

We need to fix the --in-memory flag to not ever access disk, and we also need to then make the --data-dir and --in-memory flags mutually exclusive.

This should be done by creating the appropriate interface for accessing the "storage" layer and ensure that all access goes through that interface.

NOTE: This is likely a prerequisite for Deterministic Simulation Testing of the whole process.