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?
: m_result_type(ResultType::MEMPOOL_ENTRY), m_vsize{vsize}, m_base_fees(fees) {}
    
    explicit MempoolAcceptResult(const uint256& other_wtxid)
        : m_result_type(ResultType::DIFFERENT_WITNESS), m_other_wtxid(other_wtxid) {}
};
/**
* Validation result for package mempool acceptance.
*/
struct PackageMempoolAcceptResult
{
    const PackageValidationState m_state;
    /**
    * Map from wtxid to finished MempoolAcceptResults. The client is responsible
    * for keeping track of the transaction objects themselves. If a result is not
    * present, it means validation was unfinished for that transaction. If there
    * was a package-wide error (see result in m_state), m_tx_results will be empty.
    */
    std::map<const uint256, const MempoolAcceptResult> m_tx_results;
    /** Package feerate, defined as the aggregated modified fees divided by the total virtual size
     * of all transactions in the package.  May be unavailable if some inputs were not available or
     * a transaction failure caused validation to terminate early. */
    std::optional<CFeeRate> m_package_feerate;
    explicit PackageMempoolAcceptResult(PackageValidationState state,
                                        std::map<const uint256, const MempoolAcceptResult>&& results)
        : m_state{state}, m_tx_results(std::move(results)) {}
    explicit PackageMempoolAcceptResult(PackageValidationState state, CFeeRate feerate,
                                        std::map<const uint256, const MempoolAcceptResult>&& results)
        : m_state{state}, m_tx_results(std::move(results)), m_package_feerate{feerate} {}
    
    explicit PackageMempoolAcceptResult(const uint256& wtxid, const MempoolAcceptResult& result)