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'm going to remove the OgmIdentityGenerator class and use directly the OgmTableGenerator as fallback. We can then create a new JIRA to address the issue related to the "identity" generation type. WDYT?