btr_cur_optimistic_insert(): Avoid unnecessary computation of rec_size.

btr_cur_pessimistic_update(): Correctly check for generic ALTER TABLE
metadata record when update an instant ADD COLUMN record.

instant_metadata(): Invoke dict_table_t::serialise_columns()
at the earliest possibility. Previously this was only invoked
during insert in dtuple_convert_big_rec().

rec_get_converted_size_comp_prefix_low(): Assume that the metadata
field will be stored externally. In dtuple_convert_big_rec() during
the rec_get_converted_size() call, it would not be there yet.