leave generating data and generating parameters out of the timing.

[whyrusleeping]

It would be better if we could generate the data and public parameters before fetching the challenge. this way we get a truer timing of the actual replication and proving portion.

Additionally, I would like to see a challenge that skips generating the snark, and just submits raw PoRep proofs, so we can entirely isolate the speed of replication (which is what I care about the minimum bound of)