Make innodb_read_only shutdown more robust

If InnoDB is started in innodb_read_only mode such that recovered incomplete transactions exist at startup (but the redo logs are clean), an assertion will fail at shutdown, because there would exist some non-prepared transactions.

InnoDB: Failing assertion: UT_LIST_GET_LEN(trx_sys->rw_trx_list) == trx_sys->n_prepared_trx

Repeating this bug requires two restarts after ensuring that some InnoDB redo log has been written, making some changes of an incomplete transaction durable:

--let $restart_parameters= --innodb-force-recovery=3

--source include/kill_and_restart_mysqld.inc

--let $restart_parameters= --innodb-read-only

--source include/restart_mysqld.inc

--let $restart_parameters=

--source include/restart_mysqld.inc