Add support for stats refresh

Created by: szarsti

… that gets fresh stats even if metrics collection is not started.

I havne't tested it yet, but I would like to hear your early feedback, whether you find this acceptable.

With this patch we could run ddb with self monitoring disabled and we would schedule flushing loop externally, by calls to

ddb-admin stats -r

Plain stats would keep existing functionality so one can still peek at last stats without triggering reset.