HADOOP-9125. LdapGroupsMapping threw CommunicationException after some idle time. Contributed by Kai Zheng.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1461864 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-YARN-474-branch-2-gd