Core/WorldSession: After sudden disconnects, session should persist for a minute

With this change, if a player alt-f4, he'll only fade from world after a minute

Behavior seen in https://www.youtube.com/watch?feature=player_detailpage&v=RSeVCw3hVgQ#t=168
I'm not completely sure about the expire time, the 1 minute was said by Shauren

If a player tries to log back in, the session will be forced out by KickPlayer()