HDFS-4877. Merging change r1490421 from trunk.

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HDFS-4867. metaSave NPEs when there are invalid blocks in repl queue. Contributed by Plamen Jeliazkov and Ravi Prakash.

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Merge r1490437 from trunk to branch-2 for YARN-761. TestNMClientAsync fails sometimes (Zhijie Shen via bikas)

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