row_undo_step(): If innodb_fast_shutdown=3 has been requested,
abort the rollback of any non-DDL transactions. Starting with
MDEV-12323, we aborted the rollback of recovered transactions. The
transactions would be rolled back on subsequent server startup.

trx_roll_report_progress(): Renamed from trx_roll_must_shutdown(),
now that the shutdown check has been moved to the only caller.

trx_commit_low(): Allow mtr=NULL for transactions that are aborted
on rollback.

trx_rollback_finish(): Clean up aborted transactions to avoid
assertion failures and memory leaks on shutdown. This code was
previously in trx_rollback_active().

trx_rollback_to_savepoint_low(), trx_rollback_for_mysql_low():
Remove some redundant assertions.