HDFS-6089. Standby NN while transitioning to active throws a connection refused error when the prior active NN process is suspended. Contributed by Jing Zhao.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.4@1579696 13f79535-47bb-0310-9956-ffa450edef68