Pull request for drive chains, possibly the biggest chang in bitcoin's history.

IMHO two issues remain:

1. From a user POV the game theory starts off stable then quickly becomes unstable when you plug in real world values. Therefore, there is a chance that participants will lose the coins they lend to the miners, due to the much weaker security of drivechains.

2. While DC has the good idea of trying to deter reorgs due to tail emissions and preventing a the need for the fork. It comes at the cost of any of the 256 coins blowing up and then spreading up to the main chain, which would require a fork too, to put right. So instead of one vector for a fork, there would be 256.

Consequently, I dont think drivechains are ready to merge. Some changes to the concept to improve the security, and to limit the risk of a fork could see it, or something like it, become part of the bitcoin eco system, in the future.

luke-jr wants to merge 17 commits into bitcoin:master from luke-jr:bip300