flag of records.  The flags may only be updated in heap-allocated
copies of records.

btr_root_raise_and_insert(),
btr_page_split_and_insert(),
btr_cur_insert_if_possible(),
btr_cur_optimistic_insert(),
btr_cur_pessimistic_insert(),
page_cur_tuple_insert(),
page_cur_insert_rec_low(): Add parameters "ext" and "n_ext".

dtuple_convert_big_rec(): Make parameter "ext" const.