HHH-9106 : Merging multiple representations of the same entity (enabled ...

...by property)

This is a pull request that creates the EntityCopyObserver implementation specified by  hibernate.event.merge.entity_copy_observer.

I don't think that the 3 implementations for EntityCopyObserver are sufficient. I think that if entity copies are going to be enabled, users will want to provide their own implementations in some cases (e.g., to allow (or disallow) entity copies for some entity classes).

For this reason, the value for hibernate.event.merge.entity_copy_observer should be a class name. I envision users wanting to start with EntityCopyAllowedLoggedObserver to see which entity classes are affected, and then ultimately extending EntityCopyAllowedObserver to suit their needs.