Bandwidth authorities should use geographically distributed bandwidth servers

When a bandwidth authority is configured with a nearby bandwidth server, it measures nearby relays really high.

Its measurements for those relays don't help "to balance load across the network such that a user can expect to have the same average stream capacity regardless of path". ​https://gitweb.torproject.org/torflow.git/tree/NetworkScanners/BwAuthority/README.spec.txt#n353

Instead, bandwidth authorities should use a geographically distributed set of bandwidth servers. We can do this by:

• each operator setting up their own set of geographically distributed bandwidth servers
• using one or more CDNs (legacy/trac#24506 (moved))
• sharing the current bandwidth servers between bandwidth authorities

Ideally, every bandwidth authority would use the same set of bandwidth servers. That would eliminate one source of bias.