Hashrate benchmark mode

[electroape]

I think it would be rather easy to implement.

What i thought that it might be very useful to benchmark what HR you get on every supported algo variants.

And even more useful to try different thread count to find out optimal value. Optimal thread count is sometimes not equal to last level cache size divided to scratchpad.

I mean, it is possible to run hash calculations without actual data from the pool ? So you don't need to actually provide donate pool for every variant.