Changes for HDFS-1623 branch.

This change list will be merged into the trunk CHANGES.txt when the HDFS-1623
branch is merged.
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HADOOP-7455. HA: Introduce HA Service Protocol Interface. (suresh)

HADOOP-7774. HA: Administrative CLI to control HA daemons. (todd)

HADOOP-7896. HA: if both NNs are in Standby mode, client needs to try failing
             back and forth several times with sleeps. (atm)