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.

To me it is something that needs further discussion on the hibernate-dev, since at the moment the different strategies delegate to OgmTableGenerator it feels a bit odd that Neo4j is the only one that is applying this kind of validation.