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.

> Omitting that label (or at least 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.

We can store the value as an additional property but labels are simpler to use and more natural in this case to me.
This has the drawback to make our code more complicated because we cannot use a single query, I guess.

I'm not sure we can avoid to store the value, Isn't it allowed to have the same segment value for two different generator keys?