Extend CollecTor to fetch recent, non-current consensuses and votes

There are discussions to extend dir-spec to serve recent, non-current consensuses and votes (legacy/trac#21378 (moved)). As of now, only the most recent, current consensus and votes are available, as well as the next ones, 5-10 minutes before they become valid.

This extension is fantastic news, because we currently rely on CollecTor to run once per hour. And if it doesn't, we'd be missing the consenus and votes from that hour. We can compensate temporary failures to some extent by having two CollecTor instances running and synchronizing missing descriptors. But ideally, we'd be able to fetch previous consensuses and votes from the Tor directories.

This is currently blocking on legacy/trac#21378 (moved). But as soon as that ticket is resolved, we can start extending CollecTor to fetch recent, non-current consensuses and votes.