kv, ycsb: add 50%ile latency numbers to load generator output

[rjnn]

Feature requested by @nstewart.

[nstewart]

Do we have the ability to calculate the mean? Or only the median?

[petermattis]

@nstewart We can calculate pretty much anything you want. We used to display median/50%-tile, but I removed it intentionally because it was obscuring how poor we were doing on 99%-tile. We can certainly add it back and/or average latency.