Lottery seed must not be previous block hash

[LWEdslev]

This makes it possible for an adversary to alter a block to give themself a better chance of winning during the next timeslot by removing/adding transactions until they have a higher chance.
A better solution is to use the hash of the previous draw, since this cannot be altered without invalidating the reward.
Even better, we use a hash of a draw which is very far back, otherwise it would be advantageous to try staking every block close to the best depth.