message: fix bug of checkpoint
diff: @@ -3466,7 +3466,18 @@ void checkpoint_thread_new(const shared_ptr<HttpServer::Response>& response,stri
  }
  else
  {
  + pthread_rwlock_rdlock(&txn_m_lock);
  + if (txn_managers.find(db_name) == txn_managers.end())
  + {
  + error = "Database transaction manager error.";
  + sendResponseMsg(1004,error,response);
  + pthread_rwlock_unlock(&txn_m_lock);
  + return;
  + }
  + shared_ptr<Txn_manager> txn_m = txn_managers[db_name];
  + pthread_rwlock_unlock(&txn_m_lock);
  + txn_m->Checkpoint();
  current_database->save();
  //NOTICE: this info is in header
  string success = "Database saved successfully.";
  