docs: public static Single<XianTransaction> getTransaction(String transactionId, boolean readOnly) {
          if (BaseLocalTransaction.getExistedLocalTrans(transactionId) != null) {
              return Single.just(BaseLocalTransaction.getExistedLocalTrans(transactionId));
          }
          if (readOnly) {
              return PoolFactory.getPool().getSlaveDatasource().getConnection()
                      .map(connection -> connection.createTransaction(transactionId));
          } else {
              return PoolFactory.getPool().getMasterDatasource().getConnection()
                      .map(connection -> connection.createTransaction(transactionId));
          }
      }