HDFS-4645.  Move from randomly generated block ID to sequentially generated block ID.  Contributed by Arpit Agarwal


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In CHANGES.txt, move HDFS-4908 to 2.1.0-beta.


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MAPREDUCE-4374. Merging change r1502046 from trunk to branch-2.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1502048 13f79535-47bb-0310-9956-ffa450edef68
updating CHANGES.txt after committing MAPREDUCE-5333,HADOOP-9661,HADOOP-9355,HADOOP-9673,HADOOP-9414,HADOOP-9416,HDFS-4797,YARN-866,YARN-736,YARN-883 to 2.1-beta branch

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YARN-569. Add support for requesting and enforcing preemption requests via
a capacity monitor. Contributed by Carlo Curino, Chris Douglas


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