Harvest job dispatching doesn't handle Harvester shutdown

The current mechanisme for updating Harvester status in the HarvestDispatcher i solely based on the status updates from the Harvesters. This means that if a Harvester is stopped, it's status will never change. So it is stopped after reporting itself available it will remain in this state even after it has been stopped.

A improvement would therefore be to have the HarvestJobManager mark harvester as unavailable if no status updates has been received in a specific timeperiod. This period needs to be based on the status update interval.