Async coeffects (acofx) for re-frame: input an event needs from the
outside world that only arrives asynchronously -- a backend request, an
IPC call, an async browser API.

Register a handler with [[reg-acofx]], or reuse an existing effect with
[[reg-acofx-by-fx]], and inject it into an event with [[inject-acofx]]
or [[inject-acofxs]]. The event handler then runs once, with every acofx
value in its coeffects, instead of a chain of load, success and
further events.

Keeps a re-frame-tasks task running while the acofxs of its event are
underway.

An event with acofxs is aborted and dispatched again once they are
done, see [[jtk-dvlp.re-frame.async-coeffects/inject-acofxs]].
Without this namespace the task of the aborted run ends with it, and
the run with the results opens a second one. Here the task is claimed
while the acofxs run and the event picks it up again when it comes
back.

Its own namespace, so re-frame-tasks is only needed by who requires it.