add config option to not rate limit authority dir conns

During today's consensus fiasco, several authorities were hitting their configured bandwidth rates. In moria1's case, we were using the default 5MB/10MB, and we were basically sustaining 5MB/s of directory output for 6+ hours. Most things weren't finishing getting written -- including votes.

weasel suggested a feature where we allow dir conns to/from authorities to go above our bandwidth limits.

I was thinking we would implement it just by making connection_is_rate_limited() say "no" for them.

but weasel suggested that we count the bytes, and reduce them from our totals, but not limit the conns. That sounds worthwhile but more complex.

On the theory that we want this hack in rather than waiting forever for the elegant solution, I convinced weasel that he should be ok with the simpler approach.

Heck, maybe rather than making it a config option, we should just make it standard behavior for authorities.