REC_INFO_DEFAULT_ROW_ADD: Renamed from REC_INFO_DEFAULT_ROW.
This is for instant ADD COLUMN only.

REC_INFO_DEFAULT_ROW_ALTER: The more generic 'default row' record,
with additional information other than instant ADD COLUMN.

dtuple_t::is_default_row(), upd_t::is_default_row(): Check if info_bits
is for the 'default row'.