branches/zip: Allow the locally stored part of externally stored columns

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.

include/row0upd.h

@@ -388,6 +388,8 @@ struct upd_node_struct{
 	dtuple_t*	row;	/* NULL, or a copy (also fields copied to
 				heap) of the row to update; this must be reset
 				to NULL after a successful update */
+	row_ext_t*	ext;	/* NULL, or prefixes of the externally
+				stored columns of the row */
 	ulint*		ext_vec;/* array describing which fields are stored
 				externally in the clustered index record of
 				row */