Auto-Reset statistics for a target if it has been down for a longer time

[Svedrin]

similar to #28

When a target such as a notebook or workstation has been offline for e.g. 3 hours, the statistics show a huge packet loss that is misleading - all packets have been lost for the last hours because the device was gone, but it's not indicative of an ongoing problem with the device. Thus it would be helpful if the statistics would auto-reset for that one target when it comes back. To make sure we don't hide any actual problems though, it must've been down for a longer period of time (e.g. 3 hours) for the auto-reset to trigger.