OGM-273 Add OgmLoder.loadEntities to load from a List of Tuple
OGM-151 Introduce EntityKeyMetadata and retrofit EntityKey
OGM-151 Introduce AssociationKeyMetadata and cache it in simple situations
OGM-228 Make EntityKeyMetadata serializable
OGM-228 Mass indexing tests
OGM-228 Add an implementation of the MassIndexer that can be used in OGM