Always build a circuit before calling ourselves "bootstrapped"?
In the current code, if we start with a complete cached set of directory documents, it's possible for a TorClient to return from bootstrap() even if there is no network connection.
That's probably not what the user wants; maybe we should also ensure that at least once circuit has built? Or possibly that should be optional.
This can be for %Arti 1.0.0: Ready for production use