Simulator improvements

[penberg]

The simulator we have right now is pretty basic. Let's improve it as follows:

• [ ] Multiple tenants with their own database, but using same I/O dispatcher. Verify that tenants read from their own database file.
• [ ] Write to the database and verify data consistency.
• [ ] Make fault injection more explicit. For example, simulate out of disk space or disk errors.
• [ ] Simulate out of memory situations. We may want explicit Allocator for this.