Implement mockable in-memory database

[afterdusk]

This would be helpful in tests to assert expected database access patterns. For example, if unexpected reads or writes are performed. See:

• https://github.com/facebook/akd/pull/328#issuecomment-1368307195
• https://github.com/facebook/akd/pull/331#issue-1517967955

[slawlor]

@afterdusk I think we now support the mocks, but I guess do we want to leave this task open to implement the test adjustments we want now?