Implement manual ack/nack

Description of changes on: https://codimd.web.cern.ch/s/r1BFhCWUP#Conclusions

• Set the connections attribute inside the listener in order to be able to ack/nack:
• Create ack_msg/nack_msg methods on the listener
• Wrap the processing of the message with a try/catch and ack/nack accordingly.