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 

-----

Now that I think about it... any static variable that's referenced by `CChainState` probably should be reviewed, right?
EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
    bool ConnectBlock(const CBlock& block, BlockValidationState& state, CBlockIndex* pindex,
                      CCoinsViewCache& view, bool fJustCheck = false) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
    
    bool DisconnectTip(BlockValidationState& state, DisconnectedBlockTransactions* disconnectpool) EXCLUSIVE_LOCKS_REQUIRED(cs_main, m_mempool->cs);
    
    /** Mark a block as precious and reorganize.
     *
     * May not be called in a validationinterface callback.
     */
    bool PreciousBlock(BlockValidationState& state, CBlockIndex* pindex)
        EXCLUSIVE_LOCKS_REQUIRED(!m_chainstate_mutex)
        LOCKS_EXCLUDED(::cs_main);
    
    bool InvalidateBlock(BlockValidationState& state, CBlockIndex* pindex)
        EXCLUSIVE_LOCKS_REQUIRED(!m_chainstate_mutex)
        LOCKS_EXCLUDED(::cs_main);
    
    void ResetBlockFailureFlags(CBlockIndex* pindex) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
    
    bool ReplayBlocks();
    
    [[nodiscard]] bool NeedsRedownload() const EXCLUSIVE_LOCKS_REQUIRED(cs_main);
    
    bool LoadGenesisBlock();
    void PruneBlockIndexCandidates();
    void UnloadBlockIndex() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
    
    bool IsInitialBlockDownload() const;