Wip 8811

Reproducing the issue was possible by hand, starting an active and a standby-replay MDS concurrently.  With this patch the issue no longer occurs and I see the log messages from the standby politely waiting for the active to rewrite the journal.

This passes the mds_journal_migration teuthology task.
[ Re-commenting; lost originals when commit IDs changed...whoops! ]

Is there some reason we can't replay the old format journals? If we get sent to active, we could do the journal rewrite at the same time we kick off our final play-thru (see MDS::standby_replay_restart() et al).