HDFS-4461.  DirectoryScanner: volume prefix takes up memory for every block that is scanned (Colin Patrick McCabe)

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MAPREDUCE-5194. Heed interrupts during Fetcher shutdown.

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Cleanup CHANGES.txt for 2.1.0-beta section and make it consistent with branch-2 and trunk.

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HDFS-4917. Merge 1494446 from branch-2.

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