HDFS-6503. Merge r1601958 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1601959 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6315. Merge r1601960 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1601963 13f79535-47bb-0310-9956-ffa450edef68
Merge r1601964 from trunk. YARN-2124. Fixed NPE in ProportionalCapacityPreemptionPolicy. Contributed by Wangda Tan

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