Config option for the distance of cubes in the machine dimension

This is for internal and testing purposes only. It is not recommended
to set this lower than 64. If you have a really laggy server you can
increase this to not have players chunkload the cubes of other machines.
Prevent stuff entering buffers on unused machines

If the machine has never been entered before and you placed items,
fluids or flux in the buffers they would get lost as soon as you
entered the machine. In fact they got stored for room number "-1",
so all newly placed machines would show the same buffers.

Fixes #10