HDFS-4583. Merged 1456052 from trunk

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HDFS-4591. HA clients can fail to fail over while Standby NN is performing long checkpoint. Contributed by Aaron T. Myers.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1456109 13f79535-47bb-0310-9956-ffa450edef68