Added cooldown to prevent menu opening twice on 1.9+ servers. Especially noticeable when a server was not reachable. (it's because on 1.9+ the player actually clicks twice on items, with the left and right hand.)
Big code optimizations, the server will no longer freeze while pinging a server because the pinger now runs in a seperate thread.
Instead of pinging the servers first then opening the menu, the menu opens first and items appear slightly later if the server does not respond quickly enough. This should feel less laggy to players.