added recover option for rabbitmq backend

In the current implementation when an exception occurs during message processing, the rabbitmq backend does not send back any response to the broker. 

If you have set the consumer to more than one iteration, this means that the message will not be re-broadcasted to the consumer until the end of the iteration cycle has been reached. 

This PR allows to configure a rabbitmq queue to send back a  `basic_recover` to the broker, which will make the message be re-broadcasted immediately.