OGM-368 Bidirectional relationship

https://hibernate.atlassian.net/browse/OGM-368

Rebased on top of: https://github.com/hibernate/hibernate-ogm/pull/326

With this patch only one relationship is going to be used per association, to implement this I have to use the table name of the association (not the collection role like before) as type of the relationship and I also have to change the ManyToManyTest (commit 69b324d). The problem is that because I'm using one relationship I don't know if I have to count it twice or just once, so I cannot implement consistently the method `TestHelper.getNumberOfAssociations(...)`. I've added an "if block" in the ManyToManyTest but let me know if you have a better idea.