OGM-292

(Based on top of https://github.com/hibernate/hibernate-ogm/pull/284, this PR is only about the [last commit](https://github.com/gunnarmorling/hibernate-ogm/compare/gunnarmorling:OGM-292~1...OGM-292)).

Based on the ML/IRC discussion, this moves all the types from a datastore module to one parent package per store, e.g. `org.hibernate.ogm.datastore.mongodb.*`.

The dialect type, the store identifier type (e.g. `MongoDB`) and the properties type (if present, e.g. `MongDBProperties`) all live at the root package of a store.