Create a benchmark script to calculate bandwidth usage retrieving descriptors and parsing them

Directory caches don't usually cache extrainfo descriptors. If sbws starts getting extrainfo descriptors, probably would need to obtain them from the directory authorities, since the the directory caches don't usually cache them.

Currently sbws is already retrieving documents from the dirauths with the FetchDirInfoEarly option.

Talking with @mikeperry and @gk, this benchmark could tells us whether it would overload dirauths.