Changes for HDFS-3042 branch.

This change list will be merged into the trunk CHANGES.txt when the HDFS-3-42
branch is merged.
------------------------------

HADOOP-8220. ZKFailoverController doesn't handle failure to become active correctly (todd)

HADOOP-8228. Auto HA: Refactor tests and add stress tests. (todd)

HADOOP-8215. Security support for ZK Failover controller (todd)

HADOOP-8245. Fix flakiness in TestZKFailoverController (todd)

HADOOP-8257. TestZKFailoverControllerStress occasionally fails with Mockito error (todd)

HADOOP-8260. Replace ClientBaseWithFixes with our own modified copy of the class (todd)

HADOOP-8246. Auto-HA: automatically scope znode by nameservice ID (todd)

HADOOP-8247. Add a config to enable auto-HA, which disables manual FailoverController (todd)

HADOOP-8306. ZKFC: improve error message when ZK is not running. (todd)