Cluster: configdigest field no longer used. Removed.
Cluster: use server.cluster_node_timeout directly.

We used to copy this value into the server.cluster structure, however this
was not necessary.

The reason why we don't directly use server.cluster->node_timeout is
that things that can be configured via redis.conf need to be directly
available in the server structure as server.cluster is allocated later
only if needed in order to reduce the memory footprint of non-cluster
instances.
Cluster: CONFIG SET cluster-node-timeout.
Cluster: reconfigure additonal slaves on failover.