HDFS-5433. When reloading fsimage during checkpointing, we should clear existing snapshottable directories. Contributed by Aaron T. Myers.

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HDFS-5432. Merging change r1537230 from trunk to branch-2.

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YARN-1357. Merging change r1537293 from trunk to branch-2.

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YARN-1358. Merging change r1537305 from trunk to branch-2.

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