[NFC] Refactor RunBenchmark() (#690)

Ok, so, i'm still trying to get to the state when it will be a trivial change to report all the separate iterations.
The old code (LHS of the diff) was rather convoluted i'd say.
I have tried to refactor it a bit into *small* logical chunks, with proper comments.
As far as i can tell, i preserved the intent of the code, what it was doing before.
The road forward still isn't clear, but i'm quite sure it's not with the old code :)