HADOOP-9948. Merging change r1526606 from trunk to branch-2.

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HDFS-5258. Merging change r1526610 from trunk to branch-2.

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HDFS-5186. Merging change r1526615 from trunk to branch-2.

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MAPREDUCE-5525. Merging change r1526624 from trunk to branch-2.

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YARN-819. ResourceManager and NodeManager should check for a minimum allowed version (Robert Parker via jeagles)

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HADOOP-9976. Different versions of avro and avro-maven-plugin (Karthik Kambatla via Sandy Ryza)

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svn merge -c 1526688 FIXES: MAPREDUCE-5513. ConcurrentModificationException in JobControl. Contributed by Robert Parker


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