tor-hsservice: Use single_attempt_timeout in backoff Runner impl.
!2024 (merged) introduced BackoffSchedule::single_attempt_timeout(), partially
implementing #1259 (closed). This completes the implementation by moving the
per-iteration timeout from the publisher reactor to backoff::Runner.
Addresses !2024 (a4f2bf5c, comment 3005097)
Note: the if should_retry block is intentionally misindented, to make
reviewing the actual changes easier. A future commit will fix the
indentation. I originally wanted to suggest reviewing this commit using
--ignore-space-change, but I found that it makes things a bit
confusing (it shows some funky indentation around the parts that have
both whitespace changes and modifications).
Closes #1259 (closed)