Investigate unifying natural-id and unique-key handling

There are a lot of similarities and "shared logic" between handling natural-ids and unique-keys within a Session. Investigate whether we can unify the handling.

As a bonus, if we are able to, we could begin to expose a load-by-unique-key much like we do with load-by-natural-id.

See:

• org.hibernate.engine.spi.EntityUniqueKey

• org.hibernate.engine.spi.PersistenceContext#getEntity(EntityUniqueKey)

• org.hibernate.engine.spi.PersistenceContext#addEntity(EntityUniqueKey, Object)

• org.hibernate.engine.internal.NaturalIdXrefDelegate