A queue that uses core.async to give you single-session service with an external event queue. Send
the queue the `evts/cancel-event` to exit your machine (without reaching the final state).

An event queue that does NOT process event delays via any kind of notification system. Delayed events will
be queued, and will be kept invisible until you ask for an event AFTER the timout of the event has occured.
This means you must manually poll this queue.

This queue DOES support any number of sessions, and maintains separate tracking for each.

This queue DOES NOT support communication with any other kind of system. Only other statecharts with session ids.

There is a helper `(next-event-time session-id q)` that will tell you when the next delayed event will be visible
(as an inst) for a given session id.

This allows you to implement your delivery mechanism in concert with your overall system.

The `process-next-event!` of this implementation processes all available events in a loop and then returns.