Avoid false positives in rtrmon when VRPs are removed

[erikrozendaal]

Track visibility of VRPs on both sides and only consider them different when the visibility does not match based on the visibility thresholds.

Also update the results even when the sources have not been modified, since the thresholds may have changed in the meantime as time passes.

[ties]

It feels like we already capture Visible with FirstSeen and LastSeen and that Visible is implicit, but my initial implementation definitely did not do this correctly - which caused alerts for us, sometimes.

[job]

Thank you for your time and contributions @erikrozendaal and @ties