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.
Cluster: time field removed from cluster messages header.

The new algorithm does not check replies time as checking for the
currentEpoch in the reply ensures that the reply is about the current
election process.