Cluster: force AUTH ACK on manual failover.

When a slave requests masters vote for a manual failover, the
REQUEST_AUTH message is flagged in a special way in order to force the
masters to give the authorization even if the master is not marked as
failing.

src/cluster.h

@@ -220,6 +220,8 @@ typedef struct {
 /* Message flags better specify the packet content or are used to
  * provide some information about the node state. */
 #define CLUSTERMSG_FLAG0_PAUSED (1<<0) /* Master paused for manual failover. */
+#define CLUSTERMSG_FLAG0_FORCEACK (1<<1) /* Give ACK to AUTH_REQUEST even if
+                                            master is up. */
 
 /* ---------------------- API exported outside cluster.c -------------------- */
 clusterNode *getNodeByQuery(redisClient *c, struct redisCommand *cmd, robj **argv, int argc, int *hashslot, int *ask);