HDFS-6731. Merging change r1612715 from trunk

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1612718 13f79535-47bb-0310-9956-ffa450edef68
YARN-2273. NPE in ContinuousScheduling thread when we lose a node. (Wei Yan via kasha)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1612721 13f79535-47bb-0310-9956-ffa450edef68