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.
This will be needed for fixing Bug #22496.
REC_MAX_INDEX_COL_LEN: New constant, copied from DICT_MAX_INDEX_COL_LEN.
row_ext_create(), row_ext_lookup(), row_ext_lookup_low(): New functions.
