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.

@DavideD, I still think this method should not be exposed here.

As [recently discussed](https://lists.jboss.org/pipermail/hibernate-dev/2014-June/011502.html) on hibernate-dev, the entire topic of id generation needs some more consideration and re-work (which should be done separately from this PR), but can we at least not further increase the confusion and not add this method?