HADOOP-10507. Merging change r1609503 from trunk to branch-2.

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HADOOP-10808. Merging change r1609509 from trunk to branch-2.

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HDFS-6631. Merging change r1609534 from trunk to branch-2.

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svn merge -c 1609543 merging from trunk to branch-2 to fix:HDFS-6647. Edit log corruption when pipeline recovery occurs for deleted file present in snapshot. Contributed by Kihwal Lee.

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Merge r1609561 from trunk. YARN-2181. Added preemption info to logs and RM web UI. Contributed by Wangda Tan

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