Changes for HDFS-3077 branch.
This will be merged into the main CHANGES.txt when the branch is merged.

HDFS-3077. Quorum-based protocol for reading and writing edit logs. Contributed by Todd Lipcon based on initial work from Brandon Li and Hari Mankude.

HDFS-3694. Fix getEditLogManifest to fetch httpPort if necessary (todd)

HDFS-3692. Support purgeEditLogs() call to remotely purge logs on JNs (todd)