extended plain wallet api for interruptin sync thread-2

src/wallet/plain_wallet_api.cpp

@@ -141,6 +141,7 @@ namespace plain_wallet
     GET_INSTANCE(pimpl, h);
     return pimpl->cancel_sync_thread();
   }
+
   std::string sync(hwallet h)
   {
     GET_INSTANCE(pimpl, h);

src/wallet/plain_wallet_api.h

@@ -21,6 +21,7 @@ namespace plain_wallet
 
   std::string start_sync_thread(hwallet h);
   std::string get_sync_status(hwallet h);
+  std::string cancel_sync_thread(hwallet h);
   std::string sync(hwallet h);
   std::string invoke(hwallet h, const std::string& params);
 }