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.

I still think it should be something like `Neo4jTestBase` rather than `...Mapping` (a mapping is something completely different to me). B we can merge it as this if you really believe that's better.