Make manual ack optional

[walro]

I am trying to use twingly-amqp in Wally, I happened to notice that we always need to ack our messages, I don't think it necessarily makes sense to require acking when working with the RPC aspect of RabbitMQ.

[walro]

This is also a problem when implementing a RPC client, the reply-to queue may not use manual ack, leads to PRECONDITION_FAILED - reply consumer cannot acknowledge (Bunny::PreconditionFailed).