Cluster: slaveof not allowed in redis.conf as well.
Cluster: don't check keys hash slots when the source is our master.

Usually we redirect clients to the right hash slot, however we don't
want to do that with our master, we want just to mirror it.
Cluster: new node field fail_time.

This is the unix time at which we set the FAIL flag for the node.
It is only valid if FAIL is set.

The idea is to use it in order to make the cluster more robust, for
instance in order to revert a FAIL state if it is long-standing but
still slots are assigned to this node, that is, no one is going to fix
these slots apparently.