now you can see the difference between SO_REUSEADDR and SO_REUSEPORT
Following test can only work on BSD-like system
1) If you run python server.py then run python server.py, then
   it will FAIL, as the SO_REUSEADDR can not work on the exactly same
   ip addres + port.
2) If you run ptython server_another.py and then run python
   server_another.py, you will SUCCEED, as SO_REUSEPORT is special
   for that! You HAVE to specify the SO_REUSEPORT for "both of the
   servers"!