Get feedback on sync state of geo-replication.
User can specify a "checkpoint", a record of the master state as of the time. User will get info when the "checkpoint is reached", ie. when data present on master at the time of the checkpoint record is synced over to slave.
Before this, we had _zero_ feedback on sync state, which is an utter antifeature.
Result (if any):
Works fine, though reports are mixed because the crucial bugfix of
http://review.gluster.org/4248 got some delay in reaching customers.