handling multiple matching constraints in DetachPartitionFinalize()

I was looking at the following code in DetachPartitionFinalize():

        /* If there's a constraint associated with the index, detach it too */
        constrOid = get_relation_idx_constraint_oid(RelationGetRelid(partRel),
                                                    idxid);

As mentioned in email thread `parenting a PK constraint to a self-FK one`, there may be multiple matching constraints, I think we should call ConstraintSetParentConstraint() for each of them.

This means adding a helper method similar to get_relation_idx_constraint_oid() which finds constraint and calls ConstraintSetParentConstraint().

I am preparing a patch.

Please let me know if my proposal makes sense.

Thanks