Cluster: slaves start failover with a small delay.

Redis Cluster can cope with a minority of nodes not informed about the
failure of a master in time for some reason (netsplit or node not
functioning properly, blocked, ...) however to wait a few seconds before
to start the failover will make most "normal" failovers simpler as the
FAIL message will propagate before the slave election happens.

src/redis.h

@@ -521,6 +521,7 @@ typedef struct redisOpArray {
 #define REDIS_CLUSTER_FAIL 1        /* The cluster can't work */
 #define REDIS_CLUSTER_NAMELEN 40    /* sha1 hex length */
 #define REDIS_CLUSTER_PORT_INCR 10000 /* Cluster port = baseport + PORT_INCR */
+#define REDIS_CLUSTER_FAILOVER_DELAY 5 /* Seconds */
 
 struct clusterNode;