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 r1537232 from branch-2 to branch-2.2

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.2@1537235 13f79535-47bb-0310-9956-ffa450edef68