replace default go resolver with caching resolver

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

util/dns/cache_test.go

@@ -0,0 +1,16 @@
+package dns
+
+import (
+	"net"
+	"testing"
+)
+
+func TestCache(t *testing.T) {
+	net.DefaultResolver = NewNetResolver(PreferIPV4(true))
+
+	addrs, err := net.LookupHost("unistack.org")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("addrs %v", addrs)
+}