Feature: more robust updater DNS resolution

- Parallel resolver to resolve multiple hosts
- Repeat resolver to repeat resolution for a single host
- Additional parameters for fault toleration
- Do not update servers if e.g. > 10% DNS resolutions failed
- resolver package in updater package

internal/updater/resolver/ips.go

@@ -0,0 +1,15 @@
+package resolver
+
+import "net"
+
+func uniqueIPsToSlice(uniqueIPs map[string]struct{}) (ips []net.IP) {
+	ips = make([]net.IP, 0, len(uniqueIPs))
+	for key := range uniqueIPs {
+		IP := net.ParseIP(key)
+		if IPv4 := IP.To4(); IPv4 != nil {
+			IP = IPv4
+		}
+		ips = append(ips, IP)
+	}
+	return ips
+}