Bw auths should group nodes by ratio (stream capacity), not consensus bw

Right now, the bw auths group nodes into "slices" of 50 nodes of similar consensus bandwidth. This is probably not the optimal grouping. They should instead be grouped by stream capacity. This is also equivalent to grouping them by the ratio of the consensus bandwidth to the descriptor bandwidth.

This would be implemented as a PathSupport.RatioPercentRestriction. We should then be able to easily switch from PercentileRestrictions to RatioPercentileRestrictions without too much issue.

This will result in faster scans and more accurate measurements.