Cluster: check that configEpoch never goes back.

Since there are ways to alter the configEpoch outside of the failover
procedure (for exampel CLUSTER SET-CONFIG-EPOCH and via the configEpoch
collision resolution algorithm), make always sure, before replacing our
configEpoch with a new one, that it is greater than the current one.

src/cluster.c

@@ -2516,7 +2516,8 @@ void clusterHandleSlaveFailover(void) {
         }
 
         /* 3) Update my configEpoch to the epoch of the election. */
-        myself->configEpoch = server.cluster->failover_auth_epoch;
+        if (myself->configEpoch < server.cluster->failover_auth_epoch)
+            myself->configEpoch = server.cluster->failover_auth_epoch;
 
         /* 4) Update state and save config. */
         clusterUpdateState();