Merge r1516331 from trunk to branch-2 for YARN-881. Priority#compareTo method seems to be wrong. (Jian He via bikas)

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Merge -c 1516337 from trunk to branch-2 to fix YARN-1082. Create base directories on HDFS after RM login to ensure RM recovery doesn't fail in secure mode. Contributed by Vinod K V.

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HDFS-5045. Merge change r1516348 from trunk.

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YARN-1082. Addendum patch.

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Merge -c 1516358 from trunk to branch-2 to fix MAPREDUCE-5468. Fix MR AM recovery for map-only jobs. Contributed by Vinod K. V.

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Merge -c 1516361 from trunk to branch-2 to fix MAPREDUCE-5475. Ensure MRClientService verifies ACLs for users. Contributed by Jason Lowe.

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