Implement transactional package upload

[LeonidVas]

Now a situation is possible when some package files were uploaded and after the operation is fail. Such behavior lead to an inconsistent state of the repository (partially uploaded package). To avoid such situation, it's necessary to implement transactional uploading of the package (the package is either fully loaded or not loaded at all).

[LeonidVas]

I think it can be done by using a "limbo" (temporary storage) on S3 where the package will be uploaded and then moved it to the repo.

[LeonidVas]

Before implementing, is needed to write an RFC describing the proposed implementation, the various cases of "something went wrong" when package uploading, and how the proposed solution would work in those cases.