Pull request: all: fix some races

Updates #3087.

Squashed commit of the following:

commit e888ce524f286f3c34e14e0086336b65a95fa020
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon May 17 16:34:10 2021 +0300

    all: fix some races

internal/home/dns.go

@@ -319,6 +319,9 @@ func applyAdditionalFiltering(clientAddr net.IP, clientID string, setts *dnsfilt
 }
 
 func startDNSServer() error {
+	config.Lock()
+	defer config.Unlock()
+
 	if isRunning() {
 		return fmt.Errorf("unable to start forwarding DNS server: Already running")
 	}