This methods is waaay too big and less readable than the original version. Also, the gem declares to support `redis >=2.4.14` while the scan method is only available since 2.8.

I would propose to wait for https://github.com/yankov/redis-migrator/pull/10/files to get merged. Then add something similar to:
```ruby
def old_cluster_keys
  @old_cluster_keys ||= begin
    if minimal_redis_version >= '2.8.0'  #minimal_redis_version could be extracted/replace 'old_nodes' methods
      Redis::KeyScanner.new(old_cluster).keys
    else
      @old_cluster.keys("*")
    end
  end
end
```