Cleanup session tracker

- call current_schema::mark_as_changed() directly
- call state_change::mark_as_changed() directly
- replaced SESSION_TRACKER_CHANGED with dummy tracker
- replaced Session_tracker::mark_as_changed() with
  State_tracker::mark_as_changed()
- hide and devirtualize original State_tracker::mark_as_changed(),
  rename it to set_changed()
- all implementations of mark_as_changed() now check is_enabled() for
  consistency
- no argument casts anymore

sql/sql_class.h

@@ -4685,9 +4685,7 @@ private:
   AUTHID invoker;
 
 public:
-#ifndef EMBEDDED_LIBRARY
   Session_tracker session_tracker;
-#endif //EMBEDDED_LIBRARY
   /*
     Flag, mutex and condition for a thread to wait for a signal from another
     thread.