[Colossus] Serve objects from pending folder

[mnaamani]

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.

[kdembler]

If an operator runs out of balance, they could build up a stack of uploaded objects that don't get accepted. If a node like that disappears for some reason, currently all those objects would get lost since other operators won't sync until object is moved to main uploads/.

[kdembler]

This is possibly blocked by the fact that storage nodes will currently not synch objects that haven't been accepted on-chain