When rolling upgrade is in progress, standby NN should create checkpoint for downgrade.

After rolling upgrade is started and checkpoint is disabled, the edit log may grow to a huge size. It is not a problem if rolling upgrade is finalized normally since NN keeps the current state in memory and it writes a new checkpoint during finalize. However, it is a problem if admin decides to downgrade. It could take a long time to apply edit log. Rollback does not have such problem.