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.
Get the number of seconds before the next retriable task is ready.
This number is specifically intended to be used as a timeout, where
`undef` means never time out.