message: Add a log message when purging the synchronizer queues.
diff: @@ -202,6 +202,7 @@ impl<I: NodeIdT, C: Context + 'static> Synchronizer<I, C> {
  /// Removes expired pending vertices from the queues, and schedules the next purge.
  pub(crate) fn purge_vertices(&mut self, now: Timestamp) {
  + info!("purging synchronizer queues");
  let oldest = now.saturating_sub(self.pending_vertex_timeout);
  self.vertices_no_deps.remove_expired(oldest);
  self.requests_sent.clear();
  