RabbitMQ: Investigate Send Confirmations

[ric-evans]

RabbitMQ does not offer bulk sending. However, when I looked into this, I did discover the rabbitmq backend does not use channel.wait_for_confirms() in send_message() despite configuring with channel.confirm_delivery().

Originally posted by @ric-evans in #58

Implementing wait_for_confirms() will inevitably slow down sending, but increase assurance. With a stable connection, we have not seen any notable cases of sends failing -- most likely this is because if the connection is unstable, then other things in the pipeline fail more drastically.

confirm_delivery() may currently be slowing down the broker.

Questions:

• Do we make this configurable, i.e. use both wait_for_confirms() and confirm_delivery(), or neither?
• Do we add a sort of quasi-batch sending where wait_for_confirms() is called on an intermittent basis?
• What happens if wait_for_confirms() raises an exception? Do we persist so we can resend?