Detects exited workers through a lapsed heartbeat. This differs
from rrq_worker_list_exited() which lists workers that have
definitely exited by checking to see if any worker that runs a
heartbeat process has not reported back in time, then marks that
worker as exited. See vignette("fault-tolerance") for details.
rrq_worker_detect_exited(controller = NULL)The controller to use. If not given (or NULL)
we'll use the controller registered with
rrq_default_controller_set().