TempfileUploadJob is not as helpful as it should be

TempfileUploadJob as it stands still requires the provider implementer to do a fair bit of work, having to implemenent cancel() and finished(), checking that the size is correct and so on.

It should be possible to give the implementer something that does the entire upload into a temp file and returns a future when that's done. With that, the implementer could just instantiate the temp file uploader and, once the file has been written, go ahead and start the upload to the cloud, without having to implement anything.

Doing this is probably quite easy; almost all of the implementation exists already in the local provider.