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:

• Reduce and cleanup writes to storage during insertion #328 (comment)
• Avoid looking up previous version of nodes that are new #331 (comment)

[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?