Make most arti-client fields reconfigurable.

This is the latest and greatest version of my big old reconfiguration branch. This time, I've made most of the parts of the configuration adjustable on the fly.

Doing this took a fair amount of refactoring along the way. There's a little more to go, as well: after this is merged, I'd like to circle back and refactor the notification mechanism in DirMgr to be more like the one in tor-events.

There isn't nearly enough in the way of tests for this code; if you can suggest any places I should add more, I'll put that on my queue as well.