Add bandwidth scanner data to NSA

In Mexico City we said we'd "add sbws data to CollecTor and to Onionoo" as part of our current roadmap. The first part, adding sbws data to CollecTor is being taken care of in legacy/trac#21378 (moved). But I wonder if we really meant sbws data or just measured bandwidths from votes to be included in Onionoo.

Adding measured bandwidths from votes requires including votes in Onionoo, which should be doable if we only consider the very latest set of votes.

Adding sbws data requires doing legacy/trac#21378 (moved) first, then extending metrics-lib, then figuring out what data to include in Onionoo, and then extending Onionoo.