OGM-392 Adding support for generated properties

Making CouchDB's "_rev" property mappable as generated property.
OGM-392 Reading back generated properties of entity documents storing association information

That's required to update the revision property of CouchDB entities whose documents contain associations
OGM-207 Avoiding usage of MongoDBDatastoreProvider#getAssociationStorage() in test helper
OGM-207 WIP