Add a consensus parameter for token bucket refill interval

When we merged the #3630 (moved) code, we changed the token bucket refill interval from 1/s to 10/s. We're now simulating whether this was a good choice in #4086 (moved).

Roger suggests that we add a consensus parameter that relays use to override their default token bucket refill interval. If we learn that 10/s wasn't the best choice, we can easily change it for all relays. Also, a consensus parameter would enable us to run experiments in the live Tor network, within reasonable bounds.

Thoughts?