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?
{
        AssertLockHeld(::cs_main);
        return m_coins_views && m_coins_views->m_cacheview;
    }
    
    
    CChain m_chain;
    /**
     * The blockhash which is the base of the snapshot this chainstate was created from.
     *
     * std::nullopt if this chainstate was not created from a snapshot.
     */
    const std::optional<uint256> m_from_snapshot_blockhash;
    
    
    bool reliesOnAssumedValid() { return m_from_snapshot_blockhash.has_value(); }