Cluster: new flag PROMOTED introduced.

A slave node set this flag for itself when, after receiving authorization
from the majority of nodes, it turns itself into a master.

At the same time now this flag is tested by nodes receiving a PING
message before reconfiguring after a failover event. This makes the
system more robust: even if currently there is no way to manually turn
a slave into a master it is possible that we'll have such a feature in
the future, or that simply because of misconfiguration a node joins the
cluster as master while others believe it's a slave. This alone is now
no longer enough to trigger reconfiguration as other nodes will check
for the PROMOTED flag.

The PROMOTED flag is cleared every time the node is turned back into a
replica of some other node.
Cluster: master node must clear its hash slots when turning into a slave.

When a master turns into a slave after a failover event, make sure to
clear the assigned slots before setting up the replication, as a slave
should never claim slots in an explicit way, but just take over the
master slots when replacing its master.