Make auxiliary worlds use the main scoreboard. Addresses BUKKIT-3984

When a world is created using our API, it does not use secondary world
server and will maintain a reference to its own scoreboard. In vanilla,
this is not an issue as there is only ever one world.

Similarly to maps, an overwrite to the scoreboard reference has been
added for when another world has been created.

This should also address BUKKIT-3982 and BUKKIT-3985
Fix boat dying twice.