Pull request: 3655 shutdown panic

Merge in DNS/adguard-home from 3655-stop-panic to master

Updates #3655.

Squashed commit of the following:

commit 5ffe5193d79a82c70e3f9f547ba52ca20f7abdeb
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Oct 6 13:06:33 2021 +0300

    dnsforward: imp code, docs

commit 3a4f04f50cd8e0d59edb9e3824f1d55bab9c73a6
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Oct 5 16:42:25 2021 +0300

    dnsforward: lock to read proxy

internal/dnsforward/dns.go

@@ -540,8 +540,16 @@ func (s *Server) processUpstream(ctx *dnsContext) (rc resultCode) {
 		}
 	}
 
-	// request was not filtered so let it be processed further
-	if ctx.err = s.dnsProxy.Resolve(d); ctx.err != nil {
+	// Process the request further since it wasn't filtered.
+
+	prx := s.proxy()
+	if prx == nil {
+		ctx.err = srvClosedErr
+
+		return resultCodeError
+	}
+
+	if ctx.err = prx.Resolve(d); ctx.err != nil {
 		return resultCodeError
 	}