Simulator improvements

[Pekka Enberg]

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.

[Joan Fontanals]

How should the tenants interact? in a multithreaded way or with concurrency as perf/latency?