row_remove_indexes_for_mysql(): Rename to row_merge_drop_indexes(),
move from row0mysql.c to row0merge.c and make the return type void.
row_merge_remove_index(): Rename to row_merge_drop_index() and make
the return type void.
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.
