Cluster: don't broadcast messages to HANDSHAKE nodes.

Also don't check for NOADDR as we check that node->link is not NULL
that's enough.
Cluster: added function to broadcast pings.

See the function top-comment for info why this is useful sometimes.
Cluster: election -> promotion in two comments.
Cluster: slave failover implemented.
Cluster: remove node from master slaves when it turns into a master.

Also, a few nearby comments improved.
Cluster: a bit more serious node role change handling.
Cluster: slaves start failover with a small delay.

Redis Cluster can cope with a minority of nodes not informed about the
failure of a master in time for some reason (netsplit or node not
functioning properly, blocked, ...) however to wait a few seconds before
to start the failover will make most "normal" failovers simpler as the
FAIL message will propagate before the slave election happens.
Cluster: log when a slave asks for failover authorization.
Version incremented to 2.9.8 after major cluster progresses.
Cluster: when failing over claim master slots.