Lots of psql duplicate key errors for `device_uniqueness` and `users_name_key`

We get thousands of these a day:

```
2019-09-02 00:00:03.009 UTC [matrix main] ERROR:  duplicate key value violates unique constraint "device_uniqueness"
2019-09-02 00:00:04.622 UTC [matrix main] ERROR:  duplicate key value violates unique constraint "users_name_key"
```

They are due (I think) to our poor man's upsert impl, which we should replace with a proper upsert.
