Add a temporary section to dir-spec reflecting what we actually do now.

      It uses the last-mapped identity value, unless all of the "binding"

      network status documents bind the name to some other identity.

5.3. Notes on what we do now.

   THIS SECTION SHOULD BE FOLDED INTO THE EARLIER SECTIONS; THEY ARE WRONG;

   THIS IS RIGHT.

   All downloaded networkstatuses are discarded once they are 10 days old (by

   published date).

   Authdirs download each others' networkstatus every

   AUTHORITY_NS_CACHE_INTERVAL minutes (currently 10).

   Directory caches download authorities' networkstatus every

   NONAUTHORITY_NS_CACHE_INTERVAL minutes (currently 10).

   Clients always try to replace any networkstatus received over

   NETWORKSTATUS_MAX_VALIDITY ago (currently 2 days). Also, when the most

   recently received networkstatus is more than

   NETWORKSTATUS_CLIENT_DL_INTERVAL (30 minutes) old, and we do not have any

   open directory connections fetching a networkstatus, clients try to

   download the networkstatus on their list after the most recently received

   networkstatus, skipping failed networkstatuses.  A networkstatus is

   "failed" if NETWORKSTATUS_N_ALLOWABLE_FAILURES (3) attempts in a row have

   all failed.

   We do not update router statuses if we have less than half of the

   networkstatuses.

   A networkstatus is "live" if it is the most recent we have received signed

   by a given trusted authority.

   A networkstatus is "recent" if it is "live" and:

       - it was received in the last DEFAULT_RUNNING_INTERVAL (currently 60

         minutes)

   OR  - it was one of the MIN_TO_INFLUENCE_RUNNING (3) most recently received

         networkstatuses.

   Authorities always believe their own opinion as to a router's status.  For

   other tors:

     - a router is valid if more than half of the live networkstatuses think

       it's valid.

     - a router is named if more than half of the live networkstatuses from

       naming authorities think it's named, and they all think it has the

       same name.

     - a router is running if more than half of the recent networkstatuses

       think it's running.

   Everyone downloads router descriptors as follows:

     - If any networkstatus lists a more recently published routerdesc with a

       different descriptor digest, and no more than

       MAX_ROUTERDESC_DOWNLOAD_FAILURES attempts to retrieve that routerdesc

       have failed, then that routerdesc is "downloadable".

     - Every DirFetchInterval, or whenever a request for routerdescs returns

       no routerdescs, we launch a set of requests for all downloadable

       routerdescs.  We divide the downloadable routerdescs into groups of no

       more than DL_PER_REQUEST, and send a request for each group to

       directory servers chosen independently.

     - We also launch a request as above when a request for routerdescs

       fails and we have no directory connections fetching routerdescs.

6. Remaining issues

   Client-knowledge partitioning is worrisome.  Most versions of this don't