HDFS-5285. Flatten INodeFile hierarchy: Add UnderContruction Feature. Merge change r1544389 from trunk.

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HDFS-5286. Merge change r1545768 from trunk.

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HDFS-5537. Merge change r1546184 from trunk.

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HDFS-5554. Merge change r1548796 from trunk.

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HDFS-5647. Merge change r1548796 from trunk.

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HDFS-5632. Merge change r1550917 from trunk.

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HDFS-5715 and HDFS-5726. Merge change r1556353 and r1556433 from trunk.

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