fix problem with dangling channels on connection restore

[przemyslawfierek-silvair]

This patch fixes problem with unexpected restoration already closed
channels (after reconnect to the AMQP broker) when the `RobustConnection`
is being used. The problem is caused by the not freeing reference to the
`aiopika.Channel._on_return` kept by the `aiormq.Channel.on_return_callbacks`,
which blocks garbage collector against free a `RobustChannel` object.

[przemyslawfierek-silvair]

To reproduce issue:

1. Open RobustConnection to a AMQP broker
2. Create RobustChannel
3. Create Queue with flags exclusive=True and auto_delete=True
4. Close the channel (note that the Queue has been automatically deleted)
5. Restart the AMQP broker
6. Note that the Channel and the Queue has been restored

[mosquito]

@przemyslawfierek-silvair thank you for your contribution. Please be patient, I have been allowing the execution of the test, but I am unlikely to be able to merge it in the near future.