srv_purge_coordinator_thread(): Wait for all purge worker threads
to actually exit. An analysis of a core dump of a hung 10.3 server
revealed that one srv_worker_thread did not exit, even though the
purge coordinator had exited. This caused kill_server_thread and
mysqld_main to wait indefinitely. The main InnoDB shutdown was
never called, because unireg_end() was never called.