Add ability to specify CPU affinity for wrk threads

[ol-damirm]

In some cases, it is necessary that the wrk2 threads run on a specific CPU. Example of usage with affinity: "wrk -t 3 -a 0,1,2 -c 100 -d 5 -R 500000 ".

If the affinity parameter is not specified, then the threads will be run as before (without any affinitization).