World's findByPaddleID function now supports const Worlds.

It now either returns a reference or a const reference (if the World is
const), throwing an exception if it can't find the right Paddle.
The setPaddle and getPaddle in the Server interface are now easier to use.

Instead there is only two overloads of getPaddle which return a
reference or a const reference depending on whether the server is
mutable or not respectively.
The PaddleGameState also uses this new code, which fixes the TODO.