Make sfen reader only stop when it's destroyed. Now it is fully RAII.

src/learn/sfen_reader.h

@@ -61,6 +61,8 @@ namespace Learner{
 
         ~SfenReader()
         {
+            stop_flag = true;
+
             if (file_worker_thread.joinable())
                 file_worker_thread.join();
         }
@@ -310,11 +312,6 @@ namespace Learner{
             }
         }
 
-        void stop()
-        {
-            stop_flag = true;
-        }
-
         void set_do_shuffle(bool v)
         {
             shuffle = v;