HDFS-5144. Document time unit to NameNodeMetrics. Contributed by Akira Ajisaka.

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HDFS-5140. Too many safemode monitor threads being created in the standby namenode causing it to fail with out of memory error. Contributed by Jing Zhao.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1518899 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-5144. Move change from 3.0 to 2.3.0 release section

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