Make the sbws node cap a proportion of the capped bandwidth

Here's a simple algorithm for capping relay bandwidths:

1. Calculate the cap as X% of the total relay bandwidth before the cap
2. If any relay is above the cap, set its bandwidth to the cap

But this algorithm calculates the cap as a proportion of the uncapped bandwidth, not the capped bandwidth. So the capped relays are actually slightly greater than X% of the capped bandwidth.

Maybe we will want to fix this issue eventually, but for the first release, the difference between X% and (X+1)% is not that important.