This commit makes the user and roles poller use a self rescheduling runnable to schedule the
next run of the poller rather than using scheduleWithFixedDelay. This is done because the
pollers perform blocking I/O operations and everything using that thread pool method runs on
the schedule thread and because of this, in certain situations this can lead to a deadlock which
will prevent the cluster from forming.
Original commit: elastic/x-pack-elasticsearch@9fd0748c8c
jaymode