This handles the echo logic, as described in
        https://github.com/raiden-network/raiden/issues/651:

            - for transfers with an amount that satisfies `amount % 3 == 0`, it sends a transfer
            with an amount of `amount - 1` back to the initiator
            - for transfers with a "lucky number" amount `amount == 7` it does not send anything
            back immediately -- after having received "lucky number transfers" from 7 different
            addresses it sends a transfer with `amount = 49` to one randomly chosen one
            (from the 7 lucky addresses)
            - consecutive entries to the lucky lottery will receive the current pool size as the
            `echo_amount`
            - for all other transfers it sends a transfer with the same `amount` back to the
            initiator

def on_transfer(self, transfer):
        """ This handles the echo logic, as described in
        https://github.com/raiden-network/raiden/issues/651:

            - for transfers with an amount that satisfies `amount % 3 == 0`, it sends a transfer
            with an amount of `amount - 1` back to the initiator
            - for transfers with a "lucky number" amount `amount == 7` it does not send anything
            back immediately -- after having received "lucky number transfers" from 7 different
            addresses it sends a transfer with `amount = 49` to one randomly chosen one
            (from the 7 lucky addresses)
            - consecutive entries to the lucky lottery will receive the current pool size as the
            `echo_amount`
            - for all other transfers it sends a transfer with the same `amount` back to the
            initiator """
        echo_amount = 0
        if transfer.amount % 3 == 0:
            log.info(
                'ECHO amount - 1',
                initiator=pex(transfer.initiator),
                amount=transfer.amount,
                identifier=transfer.identifier,
            )
            echo_amount = transfer.amount - 1

        elif transfer.amount == 7:
            log.info(
                'ECHO lucky number draw',
                initiator=pex(transfer.initiator),
                amount=transfer.amount,
                identifier=transfer.identifier,
                poolsize=self.lottery_pool.qsize(),
            )

            # obtain a local copy of the pool
            pool = self.lottery_pool.copy()
            tickets = [pool.get() for _ in range(pool.qsize())]
            assert pool.empty()
            del pool

            if any(ticket.initiator == transfer.initiator for ticket in tickets):
                assert transfer not in tickets
                log.debug(
                    'duplicate lottery entry',
                    initiator=pex(transfer.initiator),
                    identifier=transfer.identifier,
                    poolsize=len(tickets),
                )
                # signal the poolsize to the participant
                echo_amount = len(tickets)

            # payout
            elif len(tickets) == 6:
                log.info('payout!')
                # reset the pool
                assert self.lottery_pool.qsize() == 6
                self.lottery_pool = Queue()
                # add new participant
                tickets.append(transfer)
                # choose the winner
                transfer = random.choice(tickets)
                echo_amount = 49
            else:
                self.lottery_pool.put(transfer)

        else:
            log.debug(
                'echo transfer received',
                initiator=pex(transfer.initiator),
                amount=transfer.amount,
                identifier=transfer.identifier,
            )
            echo_amount = transfer.amount

        if echo_amount:
            log.debug(
                'sending echo transfer',
                target=pex(transfer.initiator),
                amount=echo_amount,
                orig_identifier=transfer.identifier,
                echo_identifier=transfer.identifier + echo_amount,
                token_address=pex(self.token_address),
                num_handled_transfers=self.num_handled_transfers + 1,
            )

            self.api.transfer(
                self.api.raiden.default_registry.address,
                self.token_address,
                echo_amount,
                transfer.initiator,
                identifier=transfer.identifier + echo_amount,
            )
        self.num_handled_transfers += 1