Exclude authorities from bandwidth scanning

The bandwidth authorities scan the bandwidth of the directory authorities, and then skip them when aggregating bandwidth for votes: https://gitweb.torproject.org/torflow.git/tree/NetworkScanners/BwAuthority/aggregate.py#n755

It would save directory authority bandwidth to skip them at the scanning stage instead.

We could do this by adding 'Authority' to the forbidden flags for entry, middle, and exit nodes here: https://gitweb.torproject.org/pytorctl.git/tree/PathSupport.py#n1151