ct_data_info() enhancement

You can now call ct_data_info() to retrieve fixed I/O fields such as object name before a a column is read. As before, the changeable fields in I/O descriptors such as pointers to text data, and length of text data are retrievable only after the column is read. This change is particularly useful when using srv_send_data() since srv_send_data() sends the row's data format before the whole row is read.