Resolve issues with duplicate spawned minecarts and platforms

"Additional thread safety" slightly increases the scope under which synchronous operations are performed, particularly in a couple of places related to creating minecart entities.

"Add cooldown (100ms) to platforms" is an additional protection to ensure that if multiple minecarts on at the same platform at the same time, we don't enter a tight loop bouncing the player between them.

In my tests this resolves the bukkit ticket referenced here: http://dev.bukkit.org/bukkit-plugins/minecartmania-reborn/tickets/87-server-freezes-when-approaching-spawn-block-incorrectly/