Dynamic submap loading

Here's the payoff, this makes map loading incremental, which means FAAST.
Additionally saving speed is improved, since it only has to write out submaps that you've been near.  Distant submaps remain untouched.
Still to be done is writing submaps out to disk and unloading them to keep memory footprint down, but that can wait for another PR.
The naming for the files is slightly changed, so this won't be compatable with the previous PR.