Cluster: FAILOVER_AUTH_REQUEST message type introduced.

This message is sent by a slave that is ready to failover its master to
other nodes to get the authorization from the majority of masters.

src/redis.h

@@ -594,6 +594,7 @@ typedef struct {
 #define CLUSTERMSG_TYPE_MEET 2          /* Meet "let's join" message */
 #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? */
 
 /* 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