OGM-207 Moving API for association storage configuration from CouchDB context types to document store context types

* Creating base classes DocumentStoreEntityContextImpl et al.
* Deriving CouchDB context classes from these
* Moving related tests to core
OGM-207 Removing AssociationKind enum as it is not required
OGM-207 Adding support for configuration association storage via annotations and API for MongoDB
OGM-207 Making association document type configurable via annotations
OGM-207 Making association document type configurable via API