Create nowplaying/albumart rate limiter based on current song

[kenellorando]

PR creates a new rate limiter that will slow down requests on the /api/nowplaying/albumart API. This API serves relatively large file image, so it's necessary to have a rate limiter mostly for the sake of data transfer volume.

This works based on current song playing. When a new song starts, a client under a single IP can request the song's album art up to 16 times over the duration of the entire song. 16 was arbitrarily selected as an amount I expect most people to be able to comfortably pull album art under normal radio usage. We'll adjust or consider parameterizing this if necessary. If the client hits the limit before the current song ends they will receive a 304 error to indicate that Cadence will not be re-transmitting the art anymore, though the album art is still the same and it is safe to use whatever it last sent.

When the current song ends, the database tracking the artwork request count is flushed and all clients reset their allowed usage of the API.