This commit adds configuration options for these values, with the
right defaults, and uses those options instead of built-in functions
to set them.
We also remove the function to extract information from chutney
directories: now that arti is configurable, it can be chutney's job
to make its own network configurations.
Among other stuff, revert parts of previous commit that introduced a
ReadableStore abstraction: there isn't a good abstraction hat covers
both Tor legacy stores and sqlite stores-- or if there is, I'll have
to find it organizally.
This isn't a final version: it probably has bugs, and it needs
documentation and more tests. But before I do those, I want to
sanity-check the API by drafting the next layer up on the stack.