Identify candidates for FallbackDir, and ship with a list of them

Back long ago, we added a feature to allow torrc to list a bunch of FallbackDir entries. These tell a client where to look for consensus directory documents, so that the clients don't just load the authorities down.

It would be good if we shipped with such a list.

One idea has been to identify directories that have:

• a stable IP:Port over a long time,
• reasonably good uptime (as a fraction)
• good bandwidth
• a contact address.

Then, contact the admins of these directory caches and ask them for permission to put them on a list.

(See also legacy/trac#8374 (moved))