o Major features:

    - Increase the base amount of time that a canonical connection
      (one that we have made to a known OR) is allowed to stay idle
      from 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 after they have been idle for a
      fixed interval, randomly add up to 50% to each connection's
      maximum timeout. 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.

    - Base connection idleness tests on the actual time elapsed since
      the connection last had circuits, not on the time when we last
      added non-padding. This change also makes it harder for an
      observer to tell when the last circuit closed by looking at when
      a connection closes. Part of a fix for ticket 6799.
      Incidentally fixes bug 12023; bugfix on 0.2.5.1-alpha.