add deletes checker post process

[ewerybody]

One of the downsides of the manifest file thing is:
If its listed in there its considered to be actually there!

If something that was uploaded by us, enlisted in the manifest is somehow removed on the target we'll still think it's there because we never do another remote scan.

We could mitigate such deletions (and changes that alter the size) with a post upload check.
This would still make the upload available asap AND fix these issues a moment later.