perf(cursored-scheduler): Retry an empty cycle once per cycle duration

[shellmayr]

• A cycle that snapshots no PKs finalizes on the tick that starts it, so the cursor returns to 0 and the next tick re-runs the cycle-start query. For a queryset that stays empty that repeats every tick interval — with the dynamic sampling scheduler's 10s tick, one full query every 10 seconds
• Adds a cursored_scheduler_empty_cycle:{name} cache entry recording when a cycle last started empty. While less than cycle_duration has passed, tick() returns early without touching the database and increments cursored_scheduler.empty_cycle_backoff
• A cycle that finds work deletes the entry, so a normally completing cycle restarts on the next tick exactly as before

Contributes to TET-2820

[linear-code]

TET-2820