Add proper sampling for estimating counts

[ramou]

We know how to do it. We've diced up the problem, done a bunch of the proofs and we've even written out how we'll do it. Let's do it and squeeze out that extra boost in performance when someone's sorting data that isn't uniformly distributed. It ain't that hard and we're already paying the cost for start/end counts because we knew we'd do this eventually.

That said, when I do this is it worth having an explicit uniform distribution version that doesn't do the start/end to squeeze out that tiny improvement from replacing a memory lookup with some basic arithmetic? I'll decide when I actually do this ticket.