[Constraints] Store checksums per constrained and not per constraint instance
saves space, opens the door to virtual constraint instances
(from redmine: issue id 2726, created on 2016-10-26, closed on 2017-01-30)
saves space, opens the door to virtual constraint instances
(from redmine: issue id 2726, created on 2016-10-26, closed on 2017-01-30)