public class UniqueConstraintHolder
UniqueConstraint annotations are handled via second pass. I do not
understand the reasons why at this time, so here I use a holder object to hold the information
needed to create the unique constraint. The ability to name it is new, and so the code used to
simply keep this as a String array (the column names).
Isn't this ultimately the same as org.hibernate.cfg.IndexOrUniqueKeySecondPass?