Refactor: database module

[AlexandraRoatis]

1. Extract database interfaces to a separate module (instead of including them in modAionBase).
2. Possibly separate the cache functionality from database implementations.
3. Add build functionality that allows the user to include only specific database implementations. With the addition of MongoDB (#731) we will have 4 different implementations available, but usually, a user opts for a single version. They should be able to create a binary that contains only that version.