Outdated cache conflicts with resumable downloads

[Michał Karnicki]

When a file is downloaded, and later U1F notices a new generation of metadata, it drops the path to the file from MetaProvider, but does not remove the old version. Initially, this was intentional. Now that we have resumable downloads, whenever generation changes, the easiest fix of the problem would be to remove the cached file immediately, so it doesn't interfere and cause a resumable download. Alternatively, leave the file, but check it's status before each opening of the file - meh.