OGM-529  Create sequences during startup

With this patch sequences are created at startup, simplifying the code a lot.
I've also split the test for the mapping of nodes and relationships, decoupling the different scenarios.

For the sequences I'm considering the claasses OgmTableGenerator, OgmIdentityGenerator, OgmSequenceGenerator. I'm not sure if I have to consider other cases as well.

Tbh. I'm against adding stuff where we know from the beginning that it doesn't really fit.

What really - from a semantically perspective - is the sequence name of an identity based generator? If identity generation isn't possbile with Neo4j in a sensible way I don't think we should pretend it is and expose awkward APIs to do so.