This is a 10.5 version of 9b750dcbd8, 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.
