code: def echo_worker(self):
          
          log.debug('echo worker', qsize=self.received_transfers.qsize())
          while self.stop_signal is None:
              if self.received_transfers.qsize() > 0:
                  transfer = self.received_transfers.get()
                  if transfer in self.seen_transfers:
                      log.debug(
                          'duplicate transfer ignored',
                          initiator=pex(transfer.initiator),
                          amount=transfer.amount,
                          identifier=transfer.identifier,
                      )
                  else:
                      self.seen_transfers.append(transfer)
                      self.greenlets.add(gevent.spawn(self.on_transfer, transfer))
              else:
                  gevent.sleep(.5)
docs: The `echo_worker` works through the `self.received_transfers` queue and spawns
          `self.on_transfer` greenlets for all not-yet-seen transfers.