Reply to PING with error when there is a MISCONF state.
SLAVEOF command refactored into a proper API.

We now have replicationSetMaster() and replicationUnsetMaster() that can
be called in other contexts (for instance Redis Cluster).
Stop writes on MISCONF only if instance is a master.

From the point of view of the slave not accepting writes from the master
can only create a bigger consistency issue.