Introduce fallback logic in `chunked_download_source` to handle memory exhaustion. When memory is low, feed the `deque` with only one uncounted buffer at a time. This allows slow but steady progress without getting stuck on the memory semaphore. Fixes: https://github.com/scylladb/scylladb/issues/25453 Fixes: https://github.com/scylladb/scylladb/issues/25262 Closes scylladb/scylladb#25452