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/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