MAPREDUCE-4949. Enable multiple pi jobs to run in parallel. (sandyr via tucu)

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merge MAPREDUCE-4946 from trunk. Fix a performance problem for large jobs by reducing the number of map completion event type conversions. Contributed by Jason Lowe


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HADOOP-9231. Merge 1437140 and 1437143 from trunk

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merge -c 1437156 from trunk to branch-2 to fix YARN-277. Use AMRMClient in DistributedShell to exemplify the approach. Contriubed by Bikas Saha

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