[FC] Add cancel() to scoped_exit

Allow scoped_exit objects to be canceled so they do not call their
callbacks when destroyed.

libraries/fc/include/fc/scoped_exit.hpp

@@ -9,12 +9,15 @@ namespace fc {
          scoped_exit( C&& c ):callback( std::forward<C>(c) ){}
          scoped_exit( scoped_exit&& mv ):callback( std::move( mv.callback ) ){}
 
+         void cancel() { canceled = true; }
+
          ~scoped_exit() {
+            if (!canceled)
                try { callback(); } catch( ... ) {}
          }
 
          scoped_exit& operator = ( scoped_exit&& mv ) {
-            callback = std::move(mv);
+            callback = std::move(mv.callback);
             return *this;
          }
       private:
@@ -22,6 +25,7 @@ namespace fc {
          scoped_exit& operator=( const scoped_exit& );
 
          Callback callback;
+         bool canceled = false;
    };
 
    template<typename Callback>