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?
/**
     * The set of all CBlockIndex entries with either BLOCK_VALID_TRANSACTIONS (for
     * itself and all ancestors) *or* BLOCK_ASSUMED_VALID (if using background
     * chainstates) and as good as our current tip or better. Entries may be failed,
     * though, and pruning nodes may be missing the data for the block.
     */
    std::set<CBlockIndex*, node::CBlockIndexWorkComparator> setBlockIndexCandidates;
    
    CCoinsViewCache& CoinsTip() EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
    {
        AssertLockHeld(::cs_main);
        assert(m_coins_views->m_cacheview);
        return *m_coins_views->m_cacheview.get();
    }
    
    CCoinsViewDB& CoinsDB() EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
    {
        AssertLockHeld(::cs_main);
        return m_coins_views->m_dbview;
    }
    
    CTxMemPool* GetMempool()
    {
        return m_mempool;
    }
    
    
    CCoinsViewErrorCatcher& CoinsErrorCatcher() EXCLUSIVE_LOCKS_REQUIRED(::cs_main)