dircache

We need a dircache, as part of Arti Relay. This is needed for core relay functionality, and for dirauth.

Subtasks:

• Figure out what this thing needs to do (for Arti Relay and for Arti dirauth)
• Design the functionality boundary, and APIs
• Design and implement internal structure

This will need a lot of cross-team input. This piece is a big unknown (and therefore represents significant planning risk) within the Arti Relay and dircache subprojects.

(This is part of O2 in P141.)

@ahf, I'm not sure how to structure this activity. I hope you don't mind me assigning this ticket to you.