Before this change, the Pikpak backend would always download
the first media item whenever possible, regardless of whether
or not it was the original contents.
Now we check the validity of a media link using the `fid`
parameter in the link URL.
Fixes#6992
Before this change the pikpak backend changed the global
--multi-thread-streams flag which wasn't desirable.
Now the machinery is in place to use the NoMultiThreading feature flag
instead.
Fixes#6915