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_test.go

@@ -0,0 +1,41 @@
+package resolver
+
+import (
+	"net"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_uniqueIPsToSlice(t *testing.T) {
+	t.Parallel()
+	testCases := map[string]struct {
+		inputIPs  map[string]struct{}
+		outputIPs []net.IP
+	}{
+		"nil": {
+			inputIPs:  nil,
+			outputIPs: []net.IP{},
+		},
+		"empty": {
+			inputIPs:  map[string]struct{}{},
+			outputIPs: []net.IP{},
+		},
+		"single IPv4": {
+			inputIPs:  map[string]struct{}{"1.1.1.1": {}},
+			outputIPs: []net.IP{{1, 1, 1, 1}},
+		},
+		"two IPv4s": {
+			inputIPs:  map[string]struct{}{"1.1.1.1": {}, "1.1.2.1": {}},
+			outputIPs: []net.IP{{1, 1, 1, 1}, {1, 1, 2, 1}},
+		},
+	}
+	for name, testCase := range testCases {
+		testCase := testCase
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+			outputIPs := uniqueIPsToSlice(testCase.inputIPs)
+			assert.Equal(t, testCase.outputIPs, outputIPs)
+		})
+	}
+}