parent_id constraint on civicrm_activity can lead to data loss

It has probably always been like this but it seems dangerous.

1. Create an activity.
2. Let's give it subject "a1" so we can tell it apart.
3. Fill out the schedule followup section at the bottom. Give it subject "a2".
4. Save.
5. Delete activity "a1".

Expected result: Just a1 is deleted.

Actual result: a1 and a2 are both deleted.

The constraint should be ON DELETE SET NULL.

As with the fix for original_id, in a large db an upgrade script might timeout or take a long time, so would need to direct the user to do it manually.