Revert _beacon_listdir thread support

The Python os.listdir function does not release the GIL so
there is no difference between threaded or not in terms of
a blocking mainloop.
Run all queries on the server before running at the client

This doubles the query time (sorry for that) but with sleeping
disks the server will block and wake them up. This solution is
far from perfect (double query time, server blocking) but it
is better than the client blocking (incl. UI).