Better model migration script

Currently we do a single non-paginated index query and then churn through the objects. If we have lots of objects (message store models, for example) this isn't good enough.
I'm wondering whether this would be clearer if it used inlineCallbacks and was just:
```
while keys_list:
    key = keys_list.pop(0)
    yield self.migrate_key(key, dry_run)
```