HDFS-5948. Merging r1568532 from trunk to branch-2.

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HDFS-5949. Merging change r1568536 from trunk

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YARN-1553. Fixed a bug with branch-2 patch that broke compilation. Contributed by Haohui Mai.


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HDFS-5698. Merge r1566359 from trunk for merging the 'Use protobuf to serialize / deserialize FSImage' feature

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HDFS-5915. Merging r1566824 from trunk to branch-2

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HDFS-5716. Merging change r1568547 from trunk

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HDFS-5847. Merging r1567812 from trunk to branch-2

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HDFS-5953. TestBlockReaderFactory fails in trunk. (Akira Ajisaka via wang)

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HDFS-5759. Merging r1568578 from trunk to branch-2

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HDFS-5698. Update CHANGES.txt in branch-2

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