Merge -c 1583241 from trunk to branch-2 to fix HDFS-4564. Ensure webhdfs returns correct HTTP response codes for denied operations. Contributed by Daryn Sharp.

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Preparing for hadoop-2.4.0 release.

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Added release notes for hadoop-2.4.0.

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HDFS-5285. svn rm hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/INodeFileUnderConstruction.java & hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/snapshot/INodeFileUnderConstructionWithSnapshot.java.

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