Validate upstream reachability on first DNS configuration (#1489)

* Test upstream DNS for availability as soon as they are received from management

* Use root zone for DNS tests

client/internal/dns/local.go

@@ -71,3 +71,5 @@ func buildRecordKey(name string, class, qType uint16) string {
 	key := fmt.Sprintf("%s_%d_%d", name, class, qType)
 	return key
 }
+
+func (d *localResolver) probeAvailability() {}