Make directory mirrors fetch from fallback directory mirrors

This is the parent task for making directory mirrors fetch from fallback directory mirrors. (legacy/trac#15775 (moved) / legacy/trac#4483 (moved) make clients fetch from fallback directory mirrors.)

I think we'll get this done in 0.2.9, if we get it done earlier, great. Clients can start fetching from fallback directory mirrors in 0.2.8 regardless.