on shutdown it might happen that
1. the server starts killing THDs
2. it sets thd->killed in srv_purge_coordinator
3. srv_purge_coordinator notices that and tells srv_workers to exit
4. srv_worker will notice that and will start exiting,
    ... assert here ...
5. server sets thd->killed in worker threads

that is, it might happen that the assert is tested before
srv_worker's THD got the kill signal.

this fixes various random crashes (on this assertion) on shutdown
in tests