We should introduce the ability for a genesis file to start from a different block height. This is to create a sense of continuity when governance is approving hard forks. It also allows those who were unaware of the upgrade to not be confused by the sudden move back to block zero. (We want to encourage protocol changes to happen, so this sort of social confusion would be a blocker on such a change)

If the block number is non-zero, then the chain should also include the header of the previous block, just to ensure that this new genesis does tie into the original genesis file. The previous block has to have 0 txs (or w/e txs were there just don't get ran), because they may be invalidated under this upgrade.

/cc @ebuchman