For instant ADD COLUMN…LAST it was possible to remove the metadata
whenever the table becomes empty. For the generic ALTER TABLE,
we must preserve the metadata on dropped or reordered columns,
because concurrent transactions are assuming that this metadata
cannot change.

rec_is_add_metadata(): Check if a record is ADD COLUMN metadata
record (not more generic instant ALTER TABLE).

btr_discard_only_page_on_level(): Preserve the generic instant
ALTER TABLE metadata.

btr_cur_optimistic_delete_func(), btr_cur_pessimistic_delete():
When the last user record is deleted, do not delete the
generic instant ALTER TABLE metadata record. Only delete
instant ADD COLUMN metadata records.