message: don't panic upon un-expected end aggregation message from the aggregator
  this can actually happen, if the coordinator's acknowledgement of the
  first message is lost for some reason, for instance
diff: @@ -315,7 +315,8 @@ impl Protocol {
  pub fn end_aggregation(&mut self, success: bool) {
  if !self.waiting_for_aggregation {
  - panic!("not waiting for aggregation");
  + error!("not waiting for aggregation");
  + return;
  }
  self.waiting_for_aggregation = false;
  if success {
  