Relation between downloads and staging

[verma]

The stager should make sure certain downloads are valid when re-applying a Tag, e.g. if the server was stopped half way while the download was in progress, the state of that download stays as :downloading, when the user restarts the server and intends to re-apply the tag, the download will not be restarted when the user re-tags the file now.

Alternatively, tag application can be deferred till the download is complete.