only add new outage entry if Device status changed

[SourceDoctor]

change of status reason on offline devices causes new outage entry without "recovering" entry before.
Outage is a outage, a change of the reason doesn't care, so a new outage only has to be added if device status has changed

happens e.G. if a problematic device is flapping between
"Device status changed to Down from snmp check." and "Device status changed to Down from icmp check."

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[murrant]

This pull request has been mentioned on LibreNMS Community. There might be relevant details there:

https://community.librenms.org/t/v1-69-release-changelog-november-2020/14124/1