Dynamically start/stop worker threads

[poef]

If the whole system is idle, there is no need to keep idle threads around, or at least no more than 1 (to avoid lag).
It would be nice to have an option to set minimum and maximum number of query threads, and start new ones if there are no more free workers and max-workers has not yet been reached. And after a time out stop idle threads untill min-workers is reached.
This allows you to keep less used stores running on a resource constrained server, without them using lots of memory most of the time.