Merge from trunk. HDFS-6620. Snapshot docs should specify about preserve options with cp command. Contributed by Stephen Chu.

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HDFS-6613. Improve logging in caching classes. (wang)

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HDFS-6493. Change dfs.namenode.startup.delay.block.deletion to second instead of millisecond. Contributed by Juan Yu.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1607733 13f79535-47bb-0310-9956-ffa450edef68
HDFS-6511. BlockManager#computeInvalidateWork() could do nothing. Contributed by Juan Yu.

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