C++20 modernization: nullptr, constexpr, smart pointers, thread safety, enum class

- Replace ~320 NULL occurrences with nullptr across 47 files (-184 net lines)
- static const -> constexpr for version, coin, utility constants
- Collapse 9 PushMessage overloads into 1 variadic template with fold expressions
- Convert boost::array -> std::array, boost::type_traits -> std:: equivalents
- pwalletMain, pScriptCheckQueue, pScriptCheckThreads -> unique_ptr
- mapOrphanBlocks values: raw CBlock* -> unique_ptr<CBlock>
- Fix data races: add locks to wallet registration, pindexBest reads, mempool exists
- pwalletdbEncryption: raw new/delete -> local unique_ptr, remove exit() calls
- PoS reward overflow: CBigNum intermediate for nCoinAge * nRewardCoinYear
- memset -> OPENSSL_cleanse for secure zeroing
- Fix const-cast UB in SetMerkleBranch
- Log silent catch(...) blocks instead of silently swallowing
- Enum class: GetMinFeeMode, WalletFeature
- std::string_view for 8 utility function parameters
- Range-for with structured bindings: 63 iterator loops modernized
- std::make_pair -> brace init: 35 sites
- Delegating constructors: CWallet, CBlockIndex
- Merkle tree caching, std::array for GetMedianTimePast
- CScript copy ctor -> = default, operator!= -> = default

src/utxosnapshot.cpp

@@ -44,7 +44,7 @@ bool DumpSnapshot(const fs::path& destPath,
         unsigned int nCollected = 0;
         while (pindex && nCollected < nHeaders) {
             CDiskBlockIndex diskindex(pindex);
-            vHeaders.push_back(std::make_pair(*pindex->phashBlock, diskindex));
+            vHeaders.push_back({*pindex->phashBlock, diskindex});
             pindex = pindex->pprev;
             nCollected++;
         }
@@ -123,7 +123,7 @@ bool DumpSnapshot(const fs::path& destPath,
     // documented on CTxDBIteratorBase guarantees a stable view of committed state.
     {
         CDataStream ssKeyPrefix(SER_DISK, CLIENT_VERSION);
-        ssKeyPrefix << std::make_pair(std::string("u"), std::make_pair(uint256(0), (unsigned int)0));
+        ssKeyPrefix << std::pair{std::string("u"), std::pair{uint256(0), (unsigned int)0}};
         std::string strPrefixBegin = ssKeyPrefix.str();
 
         auto it = txdbRead.NewIterator();