ENG-5534: Watch for new partitions on newly promoted leader appointer.

If the leader appointer was promoted because of a node failure, it didn't setup
a watch on the partitions ZK dir. New partitions will not be noticed on elastic
join.
Correct unit of time in rebalance progress log message.
Merge pull request #982 from VoltDB/ENG-5534

ENG-5534: watch for new partitions in promoted LeaderAppointer