DB connection for request is not released when there are more than one database

With `use_connection_for_request=True` and two DBs:

```
db1.init_app(app)
db2.init_app(app)
```

And in the fastapi router, query db1 and db2.

1. Call the api
2. Change file contents to make it reload
3. Try exiting

Then you'll see the server gets stuck.

