update mission cancel

src/mission/mission_queue.hpp

@@ -29,6 +29,7 @@ public:
   bool reorder(const nlohmann::json& ordered_ids, std::string& err);
   bool pause(std::string& err);
   bool resume(std::string& err);
+  bool cancel(std::string& err);
 
 private:
   enum class LoopControl { None, Break, Continue };
@@ -42,6 +43,7 @@ private:
   std::atomic<bool> stop_{false};
   std::atomic<bool> wake_{false};
   std::atomic<bool> paused_{false};
+  std::atomic<bool> cancel_{false};
 
   void load();
   void saveUnlocked() const;