Add option for non-uniform key distribution

I suggest adding an option to use a [Zipf](https://en.wikipedia.org/wiki/Zipf%27s_law) distribution to sample the keys. It might give a more realistic work load for heavy contention applications as many real word datasets follow it.

The rand_distr crate has an implementation that is not yet released [here](https://github.com/rust-random/rand/blob/master/rand_distr/src/zipf.rs).