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)

HDFS-2591. MiniDFSCluster support to mix and match federation with HA (todd)

HDFS-1975. Support for sharing the namenode state from active to standby. (jitendra, atm, todd)

HDFS-1971. Send block report from datanode to both active and standby namenodes. (sanjay, todd via suresh)

HDFS-2616. Change DatanodeProtocol#sendHeartbeat() to return HeartbeatResponse. (suresh)

HDFS-2622. Fix TestDFSUpgrade in HA branch. (todd)

HDFS-2612. Handle refreshNameNodes in federated HA clusters (todd)

HDFS-2623. Add test case for hot standby capability (todd)

HDFS-2626. BPOfferService.verifyAndSetNamespaceInfo needs to be synchronized (todd)

HDFS-2624. ConfiguredFailoverProxyProvider doesn't correctly stop ProtocolTranslators (todd)

HDFS-2625. TestDfsOverAvroRpc failing after introduction of HeartbeatResponse type (todd)