Changes for HDFS-1623 branch.

This change list will be merged into the trunk CHANGES.txt when the HDFS-1623
branch is merged.
------------------------------

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)

HDFS-2301. Start/stop appropriate namenode services when transition to active and standby states. (suresh)

HDFS-2231. Configuration changes for HA namenode. (suresh)

HDFS-2418. Change ConfiguredFailoverProxyProvider to take advantage of HDFS-2231. (atm)

HDFS-2393. Mark appropriate methods of ClientProtocol with the idempotent annotation. (atm)

HDFS-2523. Small NN fixes to include HAServiceProtocol and prevent NPE on shutdown. (todd)

HDFS-2577. NN fails to start since it tries to start secret manager in safemode. (todd)

HDFS-2582. Scope dfs.ha.namenodes config by nameservice (todd)