MAPREDUCE-5744. Job hangs because RMContainerAllocator.preemptReduce() violates the comparator contract (Gera Shegalov via kasha)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1565479 13f79535-47bb-0310-9956-ffa450edef68