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?
* Check if transaction will be final in the next block to be created.
 */
bool CheckFinalTxAtTip(const CBlockIndex& active_chain_tip, const CTransaction& tx) EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
/**
 * Check if transaction will be BIP68 final in the next block to be created on top of tip.
 * @param[in]   tip             Chain tip to check tx sequence locks against. For example,
 *                              the tip of the current active chain.
 * @param[in]   coins_view      Any CCoinsView that provides access to the relevant coins for
 *                              checking sequence locks. For example, it can be a CCoinsViewCache
 *                              that isn't connected to anything but contains all the relevant
 *                              coins, or a CCoinsViewMemPool that is connected to the
 *                              mempool and chainstate UTXO set. In the latter case, the caller is
 *                              responsible for holding the appropriate locks to ensure that
 *                              calls to GetCoin() return correct coins.
 * Simulates calling SequenceLocks() with data from the tip passed in.
 * Optionally stores in LockPoints the resulting height and time calculated and the hash
 * of the block needed for calculation or skips the calculation and uses the LockPoints
 * passed in for evaluation.
 * The LockPoints should not be considered valid if CheckSequenceLocksAtTip returns false.
 */
bool CheckSequenceLocksAtTip(CBlockIndex* tip,
                        const CCoinsView& coins_view,
                        const CTransaction& tx,
                        LockPoints* lp = nullptr,
                        bool useExistingLockPoints = false);
/**
 * Closure representing one script verification
 * Note that this stores references to the spending transaction
 */
class CScriptCheck