Introduce third approach: Keep old chain running

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In that case you want to avoid that blocks intermingle between the old and the new network, which is possible since they share the same history. If not taken care of, new blocks from the old chain after the fork could enter the new chain and cause problems.

1. Specify exact block when snapshot is taken and transferred to new network
2. Prepare a chainspec with proper protocolId and badBlocks feature and share in time