ResultSet::TaskRetries: add missing method, get_retryable_task

Yet again, manual testing is proving to be insufficient. I'm pretty sure I wrote this code but lost it in a rebase, or perhaps the switch to result classes.

At any rate, this implements the actual "fetch a retry row and run it" for the hydra-notify daemon.

Tested by hand.

Created by  Graham Christensen  on September 7, 2021
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