Document conditional consensus urls in dirspec

   The most recent v3 consensus should be available at:

      http://<hostname>/tor/status-vote/current/consensus.z

   Starting with Tor version 0.2.1.1-alpha is also available at:

      http://<hostname>/tor/status-vote/current/consensus/<F1>+<F2>+<F3>.z

   Where F1, F2, etc. are authority identity fingerprints the client trusts.

   Servers will only return a consensus if more than half of the requested

   authorities have signed the document, otherwise a 404 error will be sent

   back.  The fingerprints can be shortened to a length of any multiple of

   two, using only the leftmost part of the encoded fingerprint.  Tor uses

   3 bytes (6 hex characters) of the fingerprint.

   Clients SHOULD sort the fingerprints in ascending order.  Server MUST

   accept any order.

   Clients SHOULD use this format when requesting consensus documents from

   directory authority servers and from caches running a version of Tor

   that is known to support this URL format.

   A concatenated set of all the current key certificates should be available

   at:

      http://<hostname>/tor/keys/all.z