Transient outages

[cchance27]

Currently, if a device is down during the run it just thinks the device doesn't exist at all and will therefor not include it in the list or in the various plugins (oxidized, librenms)

We should cache past successes and only drop it from cache after a certain TTL, that way we don't remove a device from oxidized if it's down during a maintenance run.

Plugins should be able to opt-in to getting cached units too or just the latest.

Librenms will only add if the device is up so no sense to try to add it if the device is down on this run.

Oxidized should keep cached devices until they are gone a few days.

DNS?