flush dns on service changes

[dovholuknf]

Right now the tunneler will flush dns on startup but it also needs to flush dns when any service changes happen affecting DNS such as when posture checks cause services to become unavailable, or available. If dns is not flushed cached when services appear, the cached results may cause problems.

[qrkourier]

I have noticed that sometimes I get a DNS result with ZET that makes it look like the service is working when the service isn't actually available. I think this issue might address the confusion of the partial failure by causing DNS to fail faster when the service isn't available.