The `REPLICAOF` command can change the replication settings of a replica on the
fly.

If a Redis server is already acting as replica, the command `REPLICAOF` NO ONE
will turn off the replication, turning the Redis server into a MASTER. In the
proper form `REPLICAOF` hostname port will make the server a replica of another
server listening at the specified hostname and port.

If a server is already a replica of some master, `REPLICAOF` hostname port will
stop the replication against the old server and start the synchronization
against the new one, discarding the old dataset.

The form `REPLICAOF` NO ONE will stop replication, turning the server into a
MASTER, but will not discard the replication. So, if the old master stops
working, it is possible to turn the replica into a master and set the
application to use this new master in read/write. Later when the other Redis
server is fixed, it can be reconfigured to work as a replica.

@return

@simple-string-reply