Add self-distribution test

[vweevers]

This is a temporary cheat to reuse the tooling we have here to test (and visualize) some of the internals (specifically keyspace which I intend to start using).

You don't have to review this, I just want you be aware this (nasty) code is here for the time being. Don't even look at the plot code 😄 (let's replace gnuplot someday - #13).

Example (click to expand)

level-bench run self-distribution memdown -b [--distribution zipfian --skew 1]
level-bench run self-distribution memdown -b [--distribution zipfian --skew=-1]
level-bench run self-distribution memdown -b [--keys seq]
level-bench plot self-distribution

Also I don't yet know where all of this is going. For example, I've yet to figure out how to use the zipfian distribution to target "randomly popular" keys like YCSB does:

Under the Zipfian distribution, items retain their popularity even as new items are inserted, whether or not the newly inserted item is popular.
Benchmarking Cloud Serving Systems with YCSB (Cooper et al., 2010)