HDFS-5842. Merge change r1562603 from trunk.

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Update HDFS and Common CHANGES.txt post branch-2.3 reswizzle.

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HDFS-5845. SecondaryNameNode dies when checkpointing with cache pools.

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HDFS-5841. Update HDFS caching documentation with new changes. (wang)

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Update CHANGES.txt to move HDFS-5842 to 2.3.0

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HADOOP-10305. Add rpc.metrics.quantile.enable and rpc.metrics.percentiles.intervals to core-default.xml. Contributed by Akira Ajisaka.

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HDFS-5492. Merging r1562683 from trunk to branch-2

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