Define a suitable difficulty function to find a nonce for POPS-VRF

[fasmat]

In #175 it is proposed to introduce a difficulty function that is used to check if a smesher has performed a suitable amount of computational cost. The difficulty function implemented in spacemeshos/post#67 is the following:

H(nonce ++ VRFpk ++ PositioningATX ) < d with d being defined as d = max(H) / N and N being the number of labels computed for the PoST initialization.

This definition for the difficulty function seems sub-optimal, since only about 63% of VRFpk will find a nonce during PoST initialization. ~ 2% of VRFpk will need to compute hashes for more than 4 times N to find a nonce that is below the difficulty threshold.

[pigmej]

That needs further discussions. Currently the ball is on @selfdual-brain side.

[pigmej]

Will be closed by: spacemeshos/post#91