Take into account track changes during lyrics fetching

[maximelouet]

When fetching lyrics for a track (which can take several seconds, even >10s sometimes), if we change the current playing track on Spotify, the new song may be reflected by getPlaybackState calls but will be reset when the getPlaybackLyrics request finishes. This will be corrected by the next getPlaybackState calls but is nonetheless quite annoying.

We should either:

1. prevent getPlaybackState requests while other requests haven't finished yet; or
2. allow getPlaybackState during getPlaybackLyrics but cancel the latter if it appears that the track changed in the meantime. We should then indeed trigger a new getPlaybackLyrics call for the new track.