fixes a bug in the db router that doesn't properly iterate through the transmission groups

If you take a look at https://github.com/rapidsms/rapidsms/blob/develop/rapidsms/router/db/router.py#L67

You'll notice this:

```python
transmissions = transmissions.filter(q).order_by('id')
```

The problem with this is that transmissions changes and during the next iteration, the method is unable to fetch the next transmission group. This problem only occurs when you're attempting to send bulk messages using different backends for the connections.

This pull request fixes that problem.