updated liteserver

- new methods for liteserver/liteclient
- added ADNL/DHT client-only work mode
- fixed crash in ADNL

crypto/vm/log.h

@@ -14,7 +14,7 @@
     You should have received a copy of the GNU Lesser General Public License
     along with TON Blockchain Library.  If not, see <http://www.gnu.org/licenses/>.
 
-    Copyright 2017-2019 Telegram Systems LLP
+    Copyright 2017-2020 Telegram Systems LLP
 */
 #pragma once
 
@@ -33,6 +33,12 @@ struct VmLog {
   td::LogOptions log_options{td::log_options};
   enum { DumpStack = 2 };
   int log_mask{1};
+  static VmLog Null() {
+    VmLog res;
+    res.log_options.level = 0;
+    res.log_mask = 0;
+    return res;
+  }
 };
 
 template <class State>