Using ControlD bootstrap DNS again

So on system where there's no available DNS, non-ControlD upstreams
could be bootstrapped like before.

While at it, also improving lookupIP to not initializing OS resolver
anymore, removing the un-necessary contention for accquiring/releasing
OS resolver mutex.

config.go

@@ -427,11 +427,18 @@ func (uc *UpstreamConfig) UID() string {
 
 // SetupBootstrapIP manually find all available IPs of the upstream.
 // The first usable IP will be used as bootstrap IP of the upstream.
+// The upstream domain will be looked up using following orders:
+//
+// - Current system DNS settings.
+// - Direct IPs table for ControlD upstreams.
+// - ControlD Bootstrap DNS 76.76.2.22
+//
+// The setup process will block until there's usable IPs found.
 func (uc *UpstreamConfig) SetupBootstrapIP() {
 	b := backoff.NewBackoff("setupBootstrapIP", func(format string, args ...any) {}, 10*time.Second)
 	isControlD := uc.IsControlD()
 	for {
-		uc.bootstrapIPs = lookupIP(uc.Domain, uc.Timeout)
+		uc.bootstrapIPs = lookupIP(uc.Domain, uc.Timeout, defaultNameservers())
 		// For ControlD upstream, the bootstrap IPs could not be RFC 1918 addresses,
 		// filtering them out here to prevent weird behavior.
 		if isControlD {
@@ -449,6 +456,11 @@ func (uc *UpstreamConfig) SetupBootstrapIP() {
 				ProxyLogger.Load().Warn().Msgf("no record found for %q, lookup from direct IP table", uc.Domain)
 			}
 		}
+		if len(uc.bootstrapIPs) == 0 {
+			ProxyLogger.Load().Warn().Msgf("no record found for %q, using bootstrap server: %s", uc.Domain, PremiumDNSBoostrapIP)
+			uc.bootstrapIPs = lookupIP(uc.Domain, uc.Timeout, []string{net.JoinHostPort(PremiumDNSBoostrapIP, "53")})
+
+		}
 		if len(uc.bootstrapIPs) > 0 {
 			break
 		}