Set the SO_REUSEADDR option on the socket (#3969)

If the server exits before the client does, the port will appear to be in use for 90 seconds unless you set this option. TCP is annoying.

celery/contrib/rdb.py

@@ -124,6 +124,7 @@ class Rdb(Pdb):
         this_port = None
         for i in range(search_limit):
             _sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+            _sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
             this_port = port + skew + i
             try:
                 _sock.bind((host, this_port))