Pull request: 1868 fix rdns

Merge in DNS/adguard-home from 1868-rdns-ipv6 to master

Updates #2943.
Updates #2704.

Squashed commit of the following:

commit 53d67ecf17ed4f9c544344288b58f3596c7246e2
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Apr 13 16:18:33 2021 +0300

    all: imp code, docs

commit 2bc15941b87f92b6fa0a7568538e02700a4385a3
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Apr 13 16:09:08 2021 +0300

    all: imp code

internal/dnsforward/clientid_test.go

@@ -16,7 +16,7 @@ import (
 // testTLSConn is a tlsConn for tests.
 type testTLSConn struct {
 	// Conn is embedded here simply to make testTLSConn a net.Conn without
-	// acctually implementing all methods.
+	// actually implementing all methods.
 	net.Conn
 
 	serverName string