Reduce CPU usage in srv_purge_shutdown.

Polling for srv_purge_should_exit() once every millisecond is enough.

storage/innobase/srv/srv0srv.cc

@@ -2329,7 +2329,7 @@ void srv_purge_shutdown()
 		while(!srv_purge_should_exit()) {
 			ut_a(!purge_sys.paused());
 			srv_wake_purge_thread_if_not_active();
-			os_thread_sleep(100);
+			os_thread_sleep(1000);
 		}
 		purge_sys.coordinator_shutdown();
 		srv_shutdown_purge_tasks();