Sanity: Interrupted uploads are not removed

[bjoerge]

If the network gets disconnected or the user abruptly closes the browser window during an upload the _upload-metadata on values is not being cleaned up and removed. This can cause dangling upload placeholders that looks like they are in progress, but that will actually never complete.

Note: this does not apply to cancelled uploads, or uploads that for some reason fails when connection is still open.

[rexxars]

I suggest we use the reflector to communicate upload status as markers instead of patching documents