Now fetchBatched maintains a sliding window of batchSize blocks in-flight.
When 75% complete, adds next chunk to maintain constant window size.
This ensures blocks are already pending or have been fetched when
StoreStream needs them.
Part of https://github.com/codex-storage/nim-codex/issues/974
Signed-off-by: Chrysostomos Nanakos <chris@include.gr>