Add load generation tool used to results.json

[hamiltont]

To future-proof the json result files, it would be good to know exactly what load generation tool was used. Currently we use wrk, older rounds used a different tool, and there's nothing saying we won't switch again to something (vegeta, httperf, etc). It should be fairly simple to drop a static string into the json results indicating what generator (and perhaps what version number) was used, and update this string each time the load generation tool is switched

[msmith-techempower]

Kicking down the road.