Track cluster topology changes

[dkropachev]

Currently s-m does not track topology changes, unless it is doing something on the cluster, then it does the following:

hosts, err := s.discoverHosts(ctx, client)
if err != nil {
	return nil, errors.Wrap(err, "discover cluster topology")
}
if err := s.setKnownHosts(c, hosts); err != nil {
	return nil, errors.Wrap(err, "update cluster")
}

Reasons to have topology change tracking:

1. There could be a case when s-m looses access to the cluster, while cluster is well and fully alive, after major topology change, see s-m can lose connection to a cluster if major topology change occurred between jobs #3454.
2. When restore is suspended and resumed it would be good for s-m to validate if any change done on the cluster that is not compatible with resuming.

These two reasons have different requirements:

1. To not to lose connection to the cluster it is enough to track ip addresses of all nodes in the cluster.
2. Restore needs to make sure that there is no nodes gone and no nodes where replaced, so you have to track node ids, ip addresses and probably file system state