Cluster: broadcast pong to other slaves in the same ring.

When we schedule a failover, broadcast a PONG to the slaves.
The other slaves that plan to get elected will do the same too, this way
it is likely that every slave will have a good picture of its own rank.

Note that this is N*N messages where N is the number of slaves for the
failing master, however usually even large clusters have many master
nodes but a limited number of replicas per node, so this is harmless.

src/cluster.c

@@ -2016,6 +2016,10 @@ void clusterHandleSlaveFailover(void) {
             server.cluster->failover_auth_time - mstime(),
             server.cluster->failover_auth_rank,
             replicationGetSlaveOffset());
+        /* Now that we have a scheduled election, broadcast our offset
+         * to all the other slaves so that they'll updated their offsets
+         * if our offset is better. */
+        clusterBroadcastPong(CLUSTER_BROADCAST_LOCAL_SLAVES);
         return;
     }