Alter flag-weight balancing equations

I have a few things I want to do to the flag-weight equations. This ticket will eventually be the parent ticket, but for now it's just a brain-dump:

1. I definitely want to correct the balancing issues discussed in legacy/trac#8240 (moved).
2. I might want to account for changes due to directory guards (legacy/trac#7757 (moved)).
3. I might want to factor in an optional Guard/Middle padding parameter. (legacy/trac#7028 (moved))
4. I might want to factor in an estimate of hidden service traffic somehow.