code: void setSavepoint(final String savepointName) {
  		if (savepointNames.contains(savepointName)) {
  			throw new IllegalStateException();
  		}
  		savepointNames.add(savepointName);
  
  		if (connection != null) {
  			try {
  				savepointMap.put(savepointName, connection.setSavepoint(savepointName));
  			} catch (SQLException e) {
  				throw new UroborosqlSQLException(e);
  			}
  		}
  	}
docs: トランザクションのセーブポイントを作成します。
  
  <pre>
  この処理は{@link Connection#setSavepoint(String)}の処理に依存します。
  </pre>
  
  @param savepointName セーブポイントの名前
  @throws SQLException SQL例外