o Major features:

    - Increate the base amount of time that a canonical connection
      (one that we have made to a known OR) is allowed to stay open
      from a 3 minutes to 15 minutes.  This leaks less information
      about when circuits have closed, and avoids unnecessary overhead
      from renegotiating connections. Part of a fix for ticket 6799.

    - Instead of closing connections at a fixed interval after their
      last circuit closed, randomly add up to 50% to each connection's
      maximum timout. This makes it harder to tell when the last
      circuit closed by looking at when a connection closes. Part of a
      fix for ticket 6799.