message: Don't overwrite existing timestamps.
diff: @@ -60,7 +60,7 @@ impl<I: NodeIdT, C: Context> PendingVertices<I, C> {
  /// Adds a holder to the vertex that satisfies `dep`.
  fn add_holder(&mut self, dep: &Dependency<C>, sender: I, time_received: Timestamp) {
  if let Some((_, holders)) = self.0.iter_mut().find(|(pvv, _)| pvv.inner().id() == *dep) {
  - holders.insert(sender, time_received);
  + holders.entry(sender).or_insert(time_received);
  }
  }
  