Cluster: set event handler in cluster bus listening socket.

The commit using listenToPort() introduced this bug by no longer
creating the event handler to handle incoming messages from the cluster
bus.

src/cluster.c

@@ -259,6 +259,15 @@ void clusterInit(void) {
         server.cfd,&server.cfd_count) == REDIS_ERR)
     {
         exit(1);
+    } else {
+        int j;
+
+        for (j = 0; j < server.cfd_count; j++) {
+            if (aeCreateFileEvent(server.el, server.cfd[j], AE_READABLE,
+                clusterAcceptHandler, NULL) == AE_ERR)
+                    redisPanic("Unrecoverable error creating Redis Cluster "
+                                "file event.");
+        }
     }
 
     /* The slots -> keys map is a sorted set. Init it. */