in the clustered index to be smaller than the indexed prefix in secondary
indexes.

row_ext_lookup(): Return NULL if the column is not stored externally.

trx_undo_rec_get_partial_row(): row_build(): Add parameter row_ext_t** ext.

row_build_index_entry(): Add the parameter row_ext_t* ext.
Invoke row_ext_lookup() to fetch prefixes of externally stored columns.

upd_node_t, undo_node_t, purge_node_t: Add the field row_ext_t* ext.