Collaborative multiprocess interaction: the cheap version

This isn't a substitute for #100, but we can make a certain amount of progress on the goal there using only the filesystem for communication.

I'm going to see how far I can get for this in %Arti 0.0.1 release: basic anonymity, since it might be really helpful for a lot of use cases.

Issues to solve:

• Sharing the cache (one writer, many readers)
• Sharing the guard state (one writer, many readers)
• Sharing the circuit timeout state (one writer, many readers)