rebased https://github.com/ethereum/go-ethereum/pull/29766 . The
downstream branch appears to have been deleted and I don't have perms to
push to that fork.
`TerminalTotalDifficultyPassed` is removed. `TerminalTotalDifficulty`
must now be non-nil, and it is expected that networks are already
merged: we can only import PoW/Clique chains, not produce blocks on
them.
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Co-authored-by: stevemilk <wangpeculiar@gmail.com>
53e3f445ed