Check Fallback Directory IPv4 and IPv6 addresses using DocTor

In legacy/trac#17158 (moved), we added a list of default fallback directories to Tor. Most clients will use these fallback directories to bootstrap in preference to the authorities.

After legacy/trac#17840 (moved) is merged, tor clients can bootstrap over IPv6. If the fallback directory has an IPv6 address, IPv6 clients will use it.

Can we check fallback IPv4 and IPv6 addresses regularly using DocTor?

It would be nice to report a summary figure, something like: "200/250 fallback directories (80%) are reachable"