Metadata loading now filters the request list down to unique file paths
that don't have metadata loaded, makes sure that another instance of the
loader isn't already loading the same files, and waits for previous
instances to complete before beginning another metadata load task. It
also will update the playlist entries for all affected tracks, even if
they were not explicitly selected for loading or reloading.
Signed-off-by: Christopher Snowhill <kode54@gmail.com>
d20df5164f