Cluster: detect cluster reconfiguration when master slots drop to 0.

The old algorithm used a PROMOTED flag and explicitly checks about
slave->master convertions. Wit the new cluster meta-data propagation
algorithm we just look at the configEpoch to check if we need to
reconfigure slots, then:

1) If a node is a master but it reaches zero served slots becuase of
reconfiguration.
2) If a node is a slave but the master reaches zero served slots because
of a reconfiguration.

We switch as a replica of the new slots owner.
Make clear that runids are not cluster node IDs.
Cluster: log message shortened.
Add REWRITE to CONFIG subcommands help message.