Improve physics thread safety;

- Error when trying to modify a collider during a world callback
- Use non-locking body interface when reading from a collider during a
  world callback to avoid deadlock
- Other Threads can still mess with colliders even while World:update is
  running in another thread, they will wait until the update is done.
- Use BodyInterface more, notably when reading collider pose info.

Remaining improvements:

There are still some places that aren't locking that maybe should be,
notably when using a body's MotionProperties?