SLAVEOF command refactored into a proper API.

We now have replicationSetMaster() and replicationUnsetMaster() that can
be called in other contexts (for instance Redis Cluster).

src/redis.h

@@ -1096,6 +1096,8 @@ void replicationCron(void);
 void replicationHandleMasterDisconnection(void);
 void replicationCacheMaster(redisClient *c);
 void resizeReplicationBacklog(long long newsize);
+void replicationSetMaster(char *ip, int port);
+void replicationUnsetMaster(void);
 
 /* Generic persistence functions */
 void startLoading(FILE *fp);