OGM 605 Fix mongodb dotted fields exception in later mongos

@gunnarmorling if you have some spare time, can you look at what I did to be sure I did not mess on some critical parts. I don't think I did but I could not test all combinations esp the separated assoc (i.e. not in-entity).

Specifically, check out @fd8133a which contains the new nesting <-> flattening logic. I think we will need to also implement that in couchdb at some point.
Also check the intermediary structure as demonstrated in @6c404df . I need to see if I can remove the `'id': { ... }` wrapper while being able to rebuild it.