Improve Async Behavior

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Some parts of the data fetching code are horrifically slow because they're performed sequentially instead of asynchronously. The most notable holdouts are as follows:

Apple Music playlist fetching

Because the API prevents fetching full song lists as extensions of a call to the playlists endpoint when fetching multiple playlists, a new request has to be made for each playlist to get its track listings. This is currently done sequentially, but with a very minor lift these tasks could be performed in parallel.