Clacks Messenger with core.async

TODO::work in progress, sorry

Pull requests are welcome

In a previous exercise we built a simple encoder/decoder to send messages via the Clacks message service (as designed by Sir Terry Pratchett, RIP).

Now we will use core.async to create a processing queue between each Clack towers, so we can then model, monitor and visualise a Clacks messenger system with multiple Clacks towers. For additional fun we will enable new Clacks towers to come on line and connect to the existing system.