One event per plant connectivity outage

When a plant goes offline you now get one alert for the outage, instead of two or none at all. It closes only once the whole plant is reachable again. Alerts for individual devices that drop out in the same moment are held back. Your plants receive this with their next agent update.

• A device that comes back while the plant is still offline is never announced on its own.
• The alert tells you how many of the plant's data loggers cannot be reached.