`:tg/owns` 'datoms' don't get retracted automatically

Consider

```
(d/transact conn [{:id 0 :array [{:id 1}]}])
(d/transact conn [{:id 0 :array' []}])
```
This is handled beautifully, except the `:tg/owns` statements on `[:id 0]` node are not retracted: `:tg/owns` is left pointing to two now non-existent nodes.

Another example:
```
(d/transact conn [{:id 0 :ent {:id 1}}])
(d/transact conn [{:id 0 :ent' nil}])
```