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.

These tests check that entities are mapped on the db as expected.The assert methods receive the expected mapping in Cypher and they fail if it does not match. That was the idea behind this name.

I don't mind to change it in any way you prefer.