Improve dispatchers that dynamically apply to core/max number of workers

[mdbergmann]

Dispatchers now use a fixed set of workers using actors with separate threads. Those workers all run right from the start and also if there is no load on the system. It would be great to be able to specify a 'core' number of workers that should run always and a 'max' number. If there is more load than the 'core' workers can handle then spawn more workers until 'max' is reached. That however implies that there is a knowledge by the dispatcher which worker is currently busy and which is idle.