For the Replication V2 delivery, we decided to implement
a heuristic where the slave to promote was chosen by the
slave which had the highest number of transactions.
This change implements the "most current slave" algorithm
as outlined in the Replication V2 spec.
Change-Id: I14c1bf45f69ac7285ff95bd4c15be9cda7a70b13
Closes-bug: #1435922