purge_node_t::in_progress: Replaces purge_node_t::done.
Only present in debug builds.

purge_node_t::start(): Moved from the start of row_purge_step().

purge_node_t::end(): Replaces row_purge_end().

trx_purge_attach_undo_recs(): Omit a check from non-debug builds.