Start archiving microdescriptors and microdescriptor consensuses

We should start archiving microdescriptors and microdescriptor consensuses, even though clients don't use them yet. We need to extend metrics-db for this, including downloading the latest microdescriptor consensus and the referenced microdescriptors every hour. We might also download all microdescriptors known to all authorities once a day, similar to what we plan to do in #2763 (moved).