Consider only including one fallback per operator

If we have a large number of fallback directory mirrors, we could make sure each operator only sees a small proportion of traffic by only including one fallback per operator.

We could identify operators by:

• Contact Info
• My Family

However, this is easy to break, because it only works for operators who correctly configure MyFamily, or use identical ContactInfo fields. (That said, these operators can still have their machines compromised.)

As a workaround, in #17158 (moved) I modified the script to output the number of fallbacks per ContactInfo. This will note in each fallback entry if an operators could see a large amount of client traffic.