start a scout if remote leadership fails

when handling a leadership change, it is possible that sirius can fail
to resolve a remote reference to a new leader. This has been observed
during rolling restarts and on commodity hardware. If this happens and a
scout is not started, the node's leader will never resolve (ie remain
unknown) and will be unable to participate properly in its cluster.
Hm, this doesn't actually test for an unresolvable leader -- this just tests that we restart a scout for phantom ballots.