Check and improve behavior when running offline

At some point before we declare %Arti 1.0.0: Ready for production use to be done, we should try running Arti with no network connection, and add tests for this. We should probably have separate tests for "no network connection", "network connection dropped after we bootstrap a directory", and "restart with bootstrapped directory".

If arti is well-behaved, it shouldn't log much at info severity or higher, and it shouldn't use much cpu. The behavior it does exhibit should be roughly what we expect.