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?
#include <optional>
#include <set>
#include <stdint.h>
#include <string>
#include <thread>
#include <utility>
#include <vector>
class Chainstate;
class CBlockTreeDB;
class CTxMemPool;
class ChainstateManager;
struct ChainTxData;
struct DisconnectedBlockTransactions;
struct PrecomputedTransactionData;
struct LockPoints;
struct AssumeutxoData;
namespace node {
class SnapshotMetadata;
} 
namespace Consensus {
struct Params;
} 
static const int MAX_SCRIPTCHECK_THREADS = 15;
static const int DEFAULT_SCRIPTCHECK_THREADS = 0;
static const int DEFAULT_STOPATHEIGHT = 0;
static const unsigned int MIN_BLOCKS_TO_KEEP = 288;
static const signed int DEFAULT_CHECKBLOCKS = 6;
static constexpr int DEFAULT_CHECKLEVEL{3};
static const uint64_t MIN_DISK_SPACE_FOR_BLOCK_FILES = 550 * 1024 * 1024;
enum class SynchronizationState {