consensus: implement Constantinople EIP 1234

[ghost]

Implements ethereum/EIPs#1234 as a part of Constantinople upgrade

cc @karalabe

[holiman]

Thanks! Instead of copy-pasting the diff calc, with a change of constant, might be nicer to have a factory create both calcdifficultyXX-methods in init. So you call it like:
calcDifficultyConstantinople = createDiffCalc(big49999)

[ghost]

@holiman Yes and you should refactor the code starting from func CalcDifficulty which will be very dirty with eip100 part 😄

[karalabe]

Closing in favor of #17675