Microdescriptors: caches fetch and server all flavors of consensus

Right now, caches only know how to download and use one single consensus flavor. We need them to be able to fetch, parse, store, and serve every flavor. Right now, that means "normal" and "microdesc".

Storing and serving are already implemented (I believe) since authorities do them. Parsing and fetching remain are partially implemented in my "microdesc_dl" branch.

See proposals 158 and 162 for more information.