Entry-point into the LambdaCD event bus.
The event-bus exists to decouple communication between various parts of LambdaCD and
allow external libraries to act on or publish events in a LambdaCD instance.
(let [subscription (subscribe ctx :some-topic)
payloads (only-payload subscription)]
(<! (go-loop 
(if-let [event (<! payloads)]
(publish! ctx :some-other-topic (:some-value event))
(unsubscribe ctx :some-topic subscription))