Allow multiple Tor instances to share directory info, guards, and who knows what else?

Frequently people want applications to run in a one-tor-per-app model. Sometimes, people want systems to run in a one-tor-per-user model. The main problem here is that some information (like choice of guards) should be consistent across all Tors, and some effort (like fetching directory information) is needlessly duplicated across all Tors.

We should look for designs that fix this.