* Clock:

      - getFramedAveragePeerClockSkew() now returns a long (ms);
        was a Long (s)
      - Implement NTP-style clock slewing so the clock is adjusted
        gradually
      - Implement clock strata so we prefer better clocks
      - Implement a timestamper in the transport so we will periodically
        update the clock even if NTP is not working
        This allows the router to converge the clock instead of simply
        hoping the first connected peer is correct.
      - Slow down NTP attempts after several consecutive failures