[bug] DNS cache doesn't survive network changes #4037
Description
When a major network change is detected, DnsResolver::reset() rebuilds the entire hickory resolver to pick up new DNS servers . This destroys the DNS cache. If DNS is temporarily unavailable during the network transition (eg WiFi→5G switches), the relay actor can't resolve the relay server hostname and all reconnection attempts fail.
We need to have a DNS cache that survives this. Potentially easier to do if we switch to simpledns and a custom resolver in #4036