Log time spent fetching playlists
Memoize models we've already translated

This saves memory, since we only have one model for each artist/album/track,
and saves *lots* of processing.

For example, with ~50 playlists of varying sizes and a fresh local cache,
fetching and translating playlists used to take about 4.2-4.6s *every* time.
With the memoization in place, the first playlist fetch takes about 3.2s, and
the subsequent fetches takes about 0.2s.