chaingen: implement DCP0001 for generator.

[dnldd]

This ports the current stake difficulty algorithm (DCP0001) to the generator.

Work towards #2090.

[rstaudt2]

This looks good to me, I went through all of the updated generator logic and compared it to the actual difficulty logic and it all looks accurate. Left a couple of really minor comments, and also noticed there is an extra space in the commit message.

[davecgh]

So, something that this does which I went back and forth a bit on is that it completely removes the old algorithm in favor of the new one which means the generator can't be used to properly test that aspect of consensus prior to activation of the DCP and testing consensus is one of the major reasons for having the generator and the fullblocktests to begin with.

However, upon closer inspection, none of the tests fail under the new algorithm because they either don't produce enough blocks to hit stake validation height, or, they maintain a steady state of votes, so the difficulty never changes.

Further, given that any alternative implementations will necessarily have to be able to sync mainnet and any issues with the old difficulty calculation code in said implementations will clearly fail if they are not correct, all new blocks are required to use the new algorithm now, and it's not a case of some easily missable corner case, this change seems acceptable.

[davecgh]

I fixed the commit message as well.