row_merge_read_clustered_index(): Check bulk_trx_id at the correct spot.

row_sel(), row_sel_try_search_shortcut(),
row_sel_try_search_shortcut_for_mysql(): Check bulk_trx_id.

ReadView::changes_visible(trx_id_t) const: New accessor for the case
where the trx_id_t is not read from a potentially corrupted index page
but directly from the memory. In this case, we can skip a sanity check.