Cluster: handle FAILOVER_AUTH_ACK messages.

That's trivial as we just need to increment the count of masters that
received with an ACK.

src/redis.h

@@ -595,6 +595,7 @@ typedef struct {
 #define CLUSTERMSG_TYPE_FAIL 3          /* Mark node xxx as failing */
 #define CLUSTERMSG_TYPE_PUBLISH 4       /* Pub/Sub Publish propagation */
 #define CLUSTERMSG_TYPE_FAILOVER_AUTH_REQUEST 5 /* May I failover? */
+#define CLUSTERMSG_TYPE_FAILOVER_AUTH_ACK 6     /* Yes, you can failover. */
 
 /* Initially we don't know our "name", but we'll find it once we connect
  * to the first node, using the getsockname() function. Then we'll use this