HDFS-4222. NN is unresponsive and loses heartbeats from DNs when configured to use LDAP and LDAP has issues. Contributed by Xiaobo Peng and Suresh Srinivas.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-1@1449539 13f79535-47bb-0310-9956-ffa450edef68