The `balancerStop` command is supposed to cancel and join ongoing migrations (it's also required by the documentation), so it should be safe to assume that no migrations take place after the command returns. However, an interleave with a stepdown can result in the command returning successfully while still one migration is proceeding.

1. A migration starts.
2. A stepdown in the config server takes place.
3. Tasks are interrupted, and among them ongoing migrations are cancelled.
4. The command balancerStop is issued. There is no ongoing migration, there is nothing to join, so the command returns immediately.
5. The interrupted migration in point 3 actually completes, despite being cancelled, and the commit happens after balancerStop has returned.