Implement sbws features in the tor authority code

sbws and torflow measure relay bandwidth, and then process the results. Some of this processing could be implemented on tor directory authorities instead. The authority implementation would be more consistent, and reduce the amount of effort required to implement new bandwidth measurement tools.

Here are some potential authority features:

• an absolute node consensus weight cap
• a relative node consensus weight cap
• the MaxAdvertisedBandwidth cap
• other post-processing in the bandwidth file spec: https://gitweb.torproject.org/torspec.git/tree/bandwidth-file-spec.txt