Task polling should start over on retries.
Check retry timers outside of task processing.

In the absence of other activity a retrying task would not go off
when its timer expired unless the suite was nudged (this is a recent-ish
bug - the ready-to-run check for clock-triggered tasks, outside of the
main task processing loop, used to check all tasks - which now (because
of retry timers) it actually should do.