Chunk-load remote vehicles if necessary.

This is almost certainly a bad (naive) implementation; however, it's a
good starting point. When the remote vehicle isn't found in the reality
bubble, we pull up the submap containing it into a temporary cache, and
we ask *that* for the vehicle.

Problems I can see right now:
    - the temp cache is never cleared
    - the temp cache is never explicitly persisted
    - vehicles in the temp cache don't generate or consume power

However, if the only way for your power grid to remain in one piece is
by transferring power through a vehicle outside the reality bubble, this
will work.

...why the hell would you put your generator so many tiles away from
where you need power? What's the use?