Fix constraint sync during migration
- restore constraints lost during merge in test schema.
- use constraint_by_eid in BeforeDeleteCWConstraintHook as done in
3.17.14 for BeforeDeleteConstrainedByHook. Fixes handling of multiple
constraints of the same type.
- make sync_schema_props_perms() delete the CWConstraint entity instead
of the constrained_by relation. In 3.19, the latter doesn't
automatically result in the former just because the relation is
composite. Simplify the constraint migration to delete all removed
constraints and recreate new ones even if they share the same type; that
optimization made the code more complicated for (AFAICT) no significant
reason.