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/sync.cpp

@@ -80,18 +80,18 @@ static void push_lock(void* c, const CLockLocation& locklocation, bool fTry)
     if (fDebug) printf("Locking: %s\n", locklocation.ToString().c_str());
     dd_mutex.lock();
 
-    (*lockstack).push_back(std::make_pair(c, locklocation));
+    (*lockstack).push_back({c, locklocation});
 
     if (!fTry) {
         for (const auto& i : (*lockstack)) {
             if (i.first == c) break;
 
-            std::pair<void*, void*> p1 = std::make_pair(i.first, c);
+            std::pair<void*, void*> p1 = {i.first, c};
             if (lockorders.count(p1))
                 continue;
             lockorders[p1] = (*lockstack);
 
-            std::pair<void*, void*> p2 = std::make_pair(c, i.first);
+            std::pair<void*, void*> p2 = {c, i.first};
             if (lockorders.count(p2))
             {
                 potential_deadlock_detected(p1, lockorders[p2], lockorders[p1]);