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/net.go

@@ -0,0 +1,17 @@
+package resolver
+
+import (
+	"context"
+	"net"
+)
+
+func newResolver(resolverAddress string) *net.Resolver {
+	d := net.Dialer{}
+	resolverAddress = net.JoinHostPort(resolverAddress, "53")
+	return &net.Resolver{
+		PreferGo: true,
+		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
+			return d.DialContext(ctx, "udp", resolverAddress)
+		},
+	}
+}