message: Add an rtt accessor.
diff: @@ -84,10 +84,12 @@ impl RttVals {
  }
  }
  - fn pto(&self) -> Duration {
  + fn rtt(&self) -> Duration {
  self.smoothed_rtt.unwrap_or(self.latest_rtt)
  - + max(4 * self.rttvar, GRANULARITY)
  - + self.max_ack_delay
  + }
  +
  + fn pto(&self) -> Duration {
  + self.rtt() + max(4 * self.rttvar, GRANULARITY) + self.max_ack_delay
  }
  }
  