fix: delete sql_event after use (#261)

### What problem were solved in this pull request?

Problem: memory leak
---------
Co-authored-by: wangyunlai <hnwyllmm@126.com>

src/observer/session/session_stage.cpp

@@ -100,8 +100,8 @@ void SessionStage::handle_request(StageEvent *event)
 
   Session::set_current_session(sev->session());
   sev->session()->set_current_request(sev);
-  SQLStageEvent *sql_event = new SQLStageEvent(sev, sql);
-  (void)handle_sql(sql_event);
+  SQLStageEvent sql_event(sev, sql);
+  (void)handle_sql(&sql_event);
 
   Communicator *communicator = sev->get_communicator();
   bool need_disconnect = false;