DATAMONGO-404 - Polishing of DBRef creation in Update clauses.

Refactored the internals of UpdateMapper to simplify the code a little. Removed the special converter in favor of handling the mapped key generation directly. This can be removed again, once DATACMNS-444 is fixed.

MetadataBackedField.getPath(String) now also rejects PersistentPropertyPaths the refer to anything else but the id property in case it traverses an association.

Changed MetadataBackedField to return the association property in calls to ….getProperty() as it is the PersistentProperty to hand to the mapping infrastructure for object conversion.

Changed MappingMongoConverter to also check, whether the given source object handed into DBRef creation is of the ID type and simply use that for DBRef creation. This allows creating DBRefs from ids as well.