MDEV-29513 avoid useless os_thread_sleep() during srv_purge_shutdown()

use waitable_task.wait() function to wait for the end of previous purge

storage/innobase/srv/srv0srv.cc

@@ -2052,7 +2052,7 @@ void srv_purge_shutdown()
 		while(!srv_purge_should_exit()) {
 			ut_a(!purge_sys.paused());
 			srv_wake_purge_thread_if_not_active();
-			os_thread_sleep(1000);
+			purge_coordinator_task.wait();
 		}
 		purge_sys.coordinator_shutdown();
 		srv_shutdown_purge_tasks();