8a017834a0
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
(cherry picked from commit dd51e50f60)
Closes scylladb/scylladb#25511