Fix lots of potential stale reads in Element.add_parent and .remove_child

Part 1/2 of #881 (closed), resolves ARKINDEX-BACKEND-SD

This fixes six possible stale reads in both add_parent and remove_child, so this affects at least CreateElement, CreateElementParent, DestroyElementParent and the move_element RQ task. There still are more elsewhere :D

See https://gitlab.com/arkindex/backend/-/issues/881#note_714675726 for details