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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HDFS-2179. Add fencing framework and mechanisms for NameNode HA. (todd)

HDFS-1974. Introduce active and standy states to the namenode. (suresh)

HDFS-2407. getServerDefaults and getStats don't check operation category (atm)

HDFS-1973. HA: HDFS clients must handle namenode failover and switch over to the new active namenode. (atm)