Cluster: log granted failover authorizations.

src/cluster.c

@@ -2367,6 +2367,8 @@ void clusterSendFailoverAuthIfNeeded(clusterNode *node, clusterMsg *request) {
     clusterSendFailoverAuth(node);
     server.cluster->lastVoteEpoch = server.cluster->currentEpoch;
     node->slaveof->voted_time = mstime();
+    redisLog(REDIS_WARNING, "Failover auth granted to %.40s for epoch %llu",
+        node->name, (unsigned long long) server.cluster->currentEpoch);
 }
 
 /* This function returns the "rank" of this instance, a slave, in the context