Do async LibraryAlbum cover fetch on a serial thread
This prevents multiple simultaneous requests to fetch the same cover,
and the async callback is still called when the cover is finally fetched
New pseudo-array controller for TableView rows. Make libraryView use it

It automatically calls the insertRow/etc. methods when it has stuff
added to it. This is far more efficient than reloadData and is a drop-in
replacement for a mutable array.