cherry-pick: all: do not check local domains when dhcp srv is off

Updates #3028.

Squashed commit of the following:

commit 49d3ca5c9de0468ccb1792e9de263fd66e30d79c
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Apr 29 15:35:32 2021 +0300

    all: do not check local domains when dhcp srv is off

internal/dhcpd/dhcpd.go

@@ -133,6 +133,7 @@ type Server struct {
 
 // ServerInterface is an interface for servers.
 type ServerInterface interface {
+	Enabled() (ok bool)
 	Leases(flags int) []Lease
 	SetOnLeaseChanged(onLeaseChanged OnLeaseChangedT)
 }
@@ -207,6 +208,11 @@ func Create(conf ServerConfig) *Server {
 	return s
 }
 
+// Enabled returns true when the server is enabled.
+func (s *Server) Enabled() (ok bool) {
+	return s.conf.Enabled
+}
+
 // server calls this function after DB is updated
 func (s *Server) onNotify(flags uint32) {
 	if flags == LeaseChangedDBStore {