MDEV-29368 Assertion `trx->mysql_thd == thd' failed in innobase_kill_query from process_timers/timer_handler and use-after-poison in innobase_kill_query

This is a 10.5 version of 9b750dcb, fix for
MDEV-23536 Race condition between KILL and transaction commit

InnoDB needs to remove trx from thd before destroying it (trx), otherwise
a concurrent KILL might get a pointer from thd to a destroyed trx.

ha_close_connection() should allow engines to clear ha_data in
hton->on close_connection(). To prevent the engine from being unloaded
while hton->close_connection() is running, we remove the lock from
ha_data and unlock the plugin manually.