DATAJPA-484 - Improved registration of JpaMetamodelMappingContext.

Instead of creating an individual instance of JpaMetamodelMappingContext per repository we now register a unique instance with access to the metamodel of the EntityManager the repositories use under "jpaMappingContext".

Weakened the contract in JpaPersistentEntityImpl to allow multiple @Id properties (in case @IdClass is used). The mapping context now also allows looking up of embeddable types as they're considered entities in the context of Spring Data mapping metadata.