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

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1518895 13f79535-47bb-0310-9956-ffa450edef68
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
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HDFS-4310-trunk-gd