[Colossus] Resumable uploads

Currently uploads are atomic - you either succeed the full upload or the full thing fails and you need to start over. This can be problematic for users with connectivity issues, on unstable connection it can be impossible to upload a larger files. Colossus should support resuming an ongoing upload - if user uploaded 50% of the file before the connection broke down, they should be able to resume from that same point and only upload the remaining 50% on retry.

This would also require support in Atlas