Allow clients joining a full server to wait for a free slot

Update the joining protocol and client connection UI so that clients attempting to join a full server are not immediately disconnected, but instead placed in a queue and added to the game as soon as a slot becomes available. The client would show a status like "Waiting for a free slot (press esc to abort)". If the wait goes on across map changes, it might clash with the current map downloading logic because the map is currently downloaded before a join is attempted, so that would need to be considered.