Pull request: all: imp cyclo in new code

Updates #2646,

Squashed commit of the following:

commit af6a6fa2b7229bc0f1c7c9083b0391a6bec7ae70
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon May 31 20:00:36 2021 +0300

    all: imp code, docs

commit 1cd4781b13e635a9e1bccb758104c1b76c78d34e
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon May 31 18:51:23 2021 +0300

    all: imp cyclo in new code

internal/dnsforward/dns_test.go

@@ -326,7 +326,7 @@ func TestServer_ProcessRestrictLocal(t *testing.T) {
 			require.Len(t, pctx.Res.Answer, tc.wantLen)
 
 			if tc.wantLen > 0 {
-				assert.Equal(t, tc.want, pctx.Res.Answer[0].Header().Name)
+				assert.Equal(t, tc.want, pctx.Res.Answer[0].(*dns.PTR).Ptr)
 			}
 		})
 	}
@@ -368,7 +368,7 @@ func TestServer_ProcessLocalPTR_usingResolvers(t *testing.T) {
 		require.Equal(t, resultCodeSuccess, rc)
 		require.NotEmpty(t, proxyCtx.Res.Answer)
 
-		assert.Equal(t, locDomain, proxyCtx.Res.Answer[0].Header().Name)
+		assert.Equal(t, locDomain, proxyCtx.Res.Answer[0].(*dns.PTR).Ptr)
 	})
 
 	t.Run("disabled", func(t *testing.T) {

internal/dnsforward/dnsforward.go

@@ -284,7 +284,7 @@ func (s *Server) Exchange(ip net.IP) (host string, err error) {
 		StartTime: time.Now(),
 	}
 
-	var resolver *proxy.Proxy = s.internalProxy
+	resolver := s.internalProxy
 	if s.subnetDetector.IsLocallyServedNetwork(ip) {
 		if !s.conf.UsePrivateRDNS {
 			return "", nil