protocol changes always retroactive on testnet

[for review, not ready to merge]

@adamkrellenstein, what do you think if here: https://github.com/CounterpartyXCP/counterpartyd/blob/retrotestnet/lib/blocks.py#L224 we propose an interactive choice (with `input()`): Quit, rollback or kickstart ?
This breaks the analogy with https://github.com/ethereum/pyethereum/blob/8761954a0f69052f9994375c77ec74e1c01a985c/pyethereum/processblock.py#L608