[Minor] refactored tests to have a base class for cleaning up and
preparing the db store
[Minor] added default IoMode, so that it must no be set on each TX
[New] added testing of the *byId() methods

Here i found a conceptual problem where the *byId() methods didn't
properly resolve ObjectRefs and add them to the TX because, we added the
ObjectRefs to the filter, which made the TX resolve PersistenContexts by
the wrong type.

Now we always add the same type of object to the ObjectFilter; the
PersistentContext. This means that when we call ObjectDao.add(T), etc.
we must already create such the context. This helps resolve another
issue at the back of my mind: We don't want to partially commit a TX if
such a basic information as the object's CTX is not available.
[New] created tests for object with no subType

This is implemented with the Book model in the test folder
[New] implemented a basic test for multiple realms (mandates)