optimize dask graph computation

[ungarj]

collect multiple tasks into larger tasks in order not to overwhelm the scheduler:

• [ ] tile batches, no overviews (concurrent.futures, dask) no dependencies
• [ ] tile batches, with overviews (dask) dependencies between tile batches; use tile outputs within one tile batch
• [ ] tile batches with preprocessing tasks (dask) dependencies between all tasks; use preprocessing output for follow up (tile) tasks