trx_t::in_rollback: Make the field available in non-debug builds.

row_undo_ins_remove_clust_rec(): On the rollback of an insert into
SYS_COLUMNS, revert instant ADD COLUMN in the cache by removing the
last column from the table and the clustered index.

page_set_instant(): Remove a too strict assertion. A rollback would
not reset the root page from FIL_PAGE_TYPE_INSTANT to FIL_PAGE_INDEX,
so the PAGE_INSTANT field could already be set.

rec_init_offsets_comp_ordinary(), rec_copy_prefix_to_buf(),
row_undo_search_clust_to_pcur(): Adjust too strict assertions.

row_search_on_row_ref(), row_undo_mod_parse_undo_rec(), row_undo_mod(),
trx_undo_update_rec_get_update(): Handle the 'default row'
as a special case.

btr_cur_trim(): In an update, trim the tuple as needed. For the
'default row', handle rollback specially. For user records, omit
fields that match the 'default row'.

btr_cur_optimistic_delete_func(), btr_cur_pessimistic_delete():
Skip locking and adaptive hash index for the 'default row'.