ERP-339
Needed to get rid of the .on_delivery() assignment and put it as part of the lambda on subscribe.  Otherwise, bunny just delivers the message and there is never acknowledgement because the lambda isn't set in time, and a race condition ensues.

Bunny documentation says you can use on_delivery, but it looks like it has issues on restart.
Merge pull request #32 from honest/ERP-339

ERP-339