Microdescriptors: clients can fetch and store microdesc consensuses and microdescs

To implement microdescriptors (see proposals 158 and 162), we'll need clients to fetch and store microdescriptors and microdescriptor consensuses. The code here will be almost completely the same as that used by caches; see tickets [legacy/trac#1754 (moved)] and [legacy/trac#1755 (moved)].