[server] make all SQL changes atomic

[waleko]

• тут достаточно update_by
  

  cavoke/server/model/sessions/game_session.cpp

  Lines 169 to 173 in 4cb9133

  	auto session = default_mp_sessions.findOne(
  	Criteria(drogon_model::cavoke_orm::Sessions::Cols::_id,
  	CompareOperator::EQ, id));
  	session.setStatus(FINISHED);
  	default_mp_sessions.update(session);

• тут придётся написать кастомную sql функцию, как в host-memorization
  

  cavoke/server/model/sessions/game_session.cpp

  Lines 146 to 156 in 4cb9133

  	auto session = default_mp_sessions.findOne(
  	Criteria(drogon_model::cavoke_orm::Sessions::Cols::_id,
  	CompareOperator::EQ, id));
  	// FIXME: not atomic, transactions perhaps or some other blocking
  	// sql-mechanism?
  	if (session.getValueOfStatus() != NOT_STARTED) {
  	throw game_session_error("session has already started");
  	}
  	session.setGameSettings(game_settings.dump());
  	session.setStatus(RUNNING);
  	default_mp_sessions.update(session);