For ENG-5579, handle race to clean up partitions during leader appointer promotion
For ENG-5579, add comments and an extra check for possible cleaned up partition
For ENG-5579, add a comment based on feedback from Ning
Merge pull request #1000 from VoltDB/ENG-5579

Eng 5579 Fix LeaderAppointer tripping over itself on promotion when joining nodes are leaving the cluster