HADOOP-9217. Merging change 1433713 from trunk

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HADOOP-9106. Merge 1433747 from trunk.

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HDFS-4288. NN accepts incremental BR as IBR in safemode. Contributed by Daryn Sharp.

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MAPREDUCE-4924. flakey test: org.apache.hadoop.mapred.TestClusterMRNotification.testMR. (rkanter via tucu)

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