Backend CCNQ4 messaging using Axon

The package includes two modules:

• backend, which is meant to run (for example) inside an abrasive-ducks process to provide endpoints for multiple clients to connect to;
• index, which is meant to run on a CCNQ4 application client.

Here an "application client" is an application such as huge-play, black-metal, etc., which typically needs to receive SUBSCRIBE messages and send NOTIFY responses to a proxy such as abrasive-ducks. The .receive method of the index modules relies on Axon's subscriber's and therefor accepts * as a pattern.