Share lazy job scheduling across dataloaders

[ydah]

This refactors lazy job scheduling shared by Dataloader and AsyncDataloader.

• Move selection and enqueueing of the next lazy depth into the base Dataloader
• Use a block for implementation-specific Fiber/Task execution
• Keep Async queue lifecycle management isolated in run_lazy_jobs
• Remove duplicated code and stale TODOs

No behavior change is intended.