(draft) possible hang on shutdown

Suggested in MDEV-13797 case:

1. Start a number of transactions that modify data.
2. In one connection, SET GLOBAL innodb_flush_log_at_trx_commit=1; and commit a small change to an InnoDB table. This ensures that the changes from the above incomplete transactions are persisted in the redo log.
3. Kill the server.
4. Restart and quickly shut down the server. It should hang on shutdown.