Preparing for release 1.2.1

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Fix up a little test failure. Contributed by atm.

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HADOOP-9504 MetricsDynamicMBeanBase has concurrency issues in createMBeanInfo. Contributed by Liang Xie and Jason Lowe.

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MAPREDUCE-5256. CombineInputFormat isn't thread safe affecting HiveServer. Contributed by Vinod Kumar Vavilapalli.

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