Cover Fetching Data Source

[Nikolaj Hald Nielsen]

> A disadvantage is the clause stated in last.fm's TOS:
> "You will not make more than 5 requests per originating IP address per
> second, averaged over a 5 minute period, without prior written
> consent."

If I am reading this right, just throttling cover queries to max 3 per
second should ensure we never run into this?

Other than that I think it sounds sane. At least it would be
interesting to do a prototype implementation to see if it works
better. Whether this is something we should do now or post 2.0.0 is
another matter entirely, I am leaning strongly to the post 2.0.0 side.

- Nikolaj