janitor may lose entire vbucket if rebalance is stopped shortly after it's moved but before vb map is updated

SUBJ. We found this as part of MB-11890.

It should be somewhat rare case but MB-11890 indicates that it's not that rare it seems. There is bug in ns_janitor that will incorrectly handle a case where old master is missing and fast-forward map chain matches vbucket states. In this case it's supposed to update vbmap to match fast-forward chain, but it fails to do it and instead keeps old chain. Which causes real vbucket on fast-forward master to be dropped and empty vbucket to be recreated on old master.