[Colossus] Serve objects from pending folder

Currently on request for an object that is still being processed in the pending folder, we return a 404.
We were concerned with what would happen if file was being streamed to the response, while the `AcceptPendingObjects` service tries to move/copy the file from pending folder to the uploads folder.

If we find that it is safe and we are okay with the resulting behaviour, and the potential latency between uploading a new object and being able to fetch it from the same node is not acceptable, then implement this change.