record is protected by a latch or a lock.

dtuple_copy(): New function: Copy a data tuple.

row_upd_replace(): New function: Apply on a row an update vector that
was built for the clustered index.  Set up a cache of externally stored
column prefixes if needed.

undo_node_t: Add the fields undo_row, undo_ext.

row_undo_search_clust_to_pcur(): Initialize undo_row and undo_ext.

row_undo_mod_upd_exist_sec(): Instead of fetching prefixes of
externally stored columns, use the undo_row and undo_ext that were
initialized in row_undo_search_clust_to_pcur().

upd_node_t: Remove the field n_ext.  Add the fields upd_row and upd_ext.

row_upd_store_row(): Initialize the upd_row and upd_ext fields of upd_node_t.

row_upd_sec_index_entry(), row_upd_clust_rec_by_insert(): Instead of
fetching prefixes of externally stored columns, use the upd_row
and upd_ext that were initialized in row_upd_store_row().