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?
std::unique_ptr<Chainstate> m_snapshot_chainstate GUARDED_BY(::cs_main);
    
    
    
    
    
    
    
    
    
    Chainstate* m_active_chainstate GUARDED_BY(::cs_main) {nullptr};
    
    
    bool m_snapshot_validated GUARDED_BY(::cs_main){false};
    CBlockIndex* m_best_invalid GUARDED_BY(::cs_main){nullptr};
    
    [[nodiscard]] bool PopulateAndValidateSnapshot(
        Chainstate& snapshot_chainstate,
        AutoFile& coins_file,
        const node::SnapshotMetadata& metadata);
    /**
     * If a block header hasn't already been seen, call CheckBlockHeader on it, ensure
     * that it doesn't descend from an invalid block, and then add it to m_block_index.
     * Caller must set min_pow_checked=true in order to add a new header to the
     * block index (permanent memory storage), indicating that the header is