Directory
*********

Directories that provide relay descriptor information. At a very high
level tor works as follows…

1. Volunteer starts a new tor relay, during which it sends a server
   descriptor to each of the directory authorities.

2. Each hour the directory authorities make a vote  that says who they
   think the active relays are in the network and some attributes
   about them.

3. The directory authorities send each other their votes, and compile
   that into the consensus. This document is very similar to the
   votes, the only difference being that the majority of the
   authorities agree upon and sign this document. The idividual relay
   entries in the vote or consensus is called router status entries.

4. Tor clients (people using the service) download the consensus from
   an authority, fallback, or other mirror to determine who the active
   relays in the network are. They then use this to construct circuits
   and use the network.

   Directory - Relay we can retrieve descriptor information from
     | |- from_cache - Provides cached information bundled with Stem.
     | +- from_remote - Downloads the latest directory information from tor.
     |
     |- Authority - Tor directory authority
     +- Fallback - Mirrors that can be used instead of the authorities

Added in version 1.7.0.
