Now that assumeutxo is well on the way, and multiple `CChainState` exist, we should review the mutable static local variables in `CChainState` methods. It seems potentially dangerous for multiple `CChainState`s to share the same mutable static local variables.

In `CChainState::FlushStateToDisk`:
https://github.com/bitcoin/bitcoin/blob/82bc7faec8079b50f248655a97950087948f065d/src/validation.cpp#L2154-L2155

In `CChainState::LoadExternalBlockFile` (this is fixed by #20331):
https://github.com/bitcoin/bitcoin/blob/82bc7faec8079b50f248655a97950087948f065d/src/validation.cpp#L4286

ping @jamesob 

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Now that I think about it... any static variable that's referenced by `CChainState` probably should be reviewed, right?
};
[[nodiscard]] bool InitScriptExecutionCache(size_t max_size_bytes);
bool CheckBlock(const CBlock& block, BlockValidationState& state, const Consensus::Params& consensusParams, bool fCheckPOW = true, bool fCheckMerkleRoot = true);
bool TestBlockValidity(BlockValidationState& state,
                       const CChainParams& chainparams,
                       Chainstate& chainstate,
                       const CBlock& block,
                       CBlockIndex* pindexPrev,
                       const std::function<NodeClock::time_point()>& adjusted_time_callback,
                       bool fCheckPOW = true,
                       bool fCheckMerkleRoot = true) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
bool HasValidProofOfWork(const std::vector<CBlockHeader>& headers, const Consensus::Params& consensusParams);
arith_uint256 CalculateHeadersWork(const std::vector<CBlockHeader>& headers);
class CVerifyDB {
public:
    CVerifyDB();
    ~CVerifyDB();
    bool VerifyDB(
        Chainstate& chainstate,
        const Consensus::Params& consensus_params,
        CCoinsView& coinsview,
        int nCheckLevel,
        int nCheckDepth) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
};
enum DisconnectResult