Microdescriptors: caches fetch and serve microdescriptors

For microdescriptors to work, clients need to be able to fetch them from the caches. Thus, caches need to fetch microdescriptors and serve them. This is mostly implemented in my "microdesc_dl" branch, but depends on [#1754 (moved)].

The "caches fetch and store microdescriptors" code will be reusable for clients.