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.

But what is the motivation/benefit of doing so? I feel like we're trying to map a concept which doesn't really fit in the context of a graph database. Our sequence nodes really represent sequences, not tables. We could store that table name it as property, but even then, what would be the benefit? 

Omitting that label (or making this a property) would allow to work with one single query and unless there is an advantage of storing the table name as label, I'd rather benefit from the improved simplicity and performance of working with one single query.