updated liteserver

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

crypto/vm/cells/CellSlice.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
 
@@ -275,6 +275,7 @@ class CellSlice : public td::CntObject {
     offs = std::min(offs, size());
     return CellSlice{*this, size() - offs, size_refs(), offs, 0};
   }
+  CellSlice clone() const;
 
  private:
   void init_bits_refs();