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/trianglesrpc.cpp

@@ -44,7 +44,7 @@ static std::string strRPCUserColonPass;
 
 const Object emptyobj;
 
-CNotificationQueue* pNotificationQueue = NULL;
+CNotificationQueue* pNotificationQueue = nullptr;
 
 void ThreadRPCServer3(void* parg);
 
@@ -367,7 +367,7 @@ const CRPCCommand *CRPCTable::operator[](string name) const
 {
     map<string, const CRPCCommand*>::const_iterator it = mapCommands.find(name);
     if (it == mapCommands.end())
-        return NULL;
+        return nullptr;
     return (*it).second;
 }
 
@@ -401,7 +401,7 @@ string rfc1123Time()
     time_t now;
     time(&now);
     struct tm* now_gmt = gmtime(&now);
-    string locale(setlocale(LC_TIME, NULL));
+    string locale(setlocale(LC_TIME, nullptr));
     setlocale(LC_TIME, "C"); // we want POSIX (aka "C") weekday/month strings
     strftime(buffer, sizeof(buffer), "%a, %d %b %Y %H:%M:%S +0000", now_gmt);
     setlocale(LC_TIME, locale.c_str());
@@ -466,7 +466,7 @@ int ReadHTTPStatus(std::basic_istream<char>& stream, int &proto,
         return HTTP_INTERNAL_SERVER_ERROR;
     proto = 0;
     const char *ver = strstr(str.c_str(), "HTTP/1.");
-    if (ver != NULL)
+    if (ver != nullptr)
         proto = atoi(ver+7);
 
     // Detect request line (GET/POST/...) vs response line (HTTP/1.x ...)
@@ -741,7 +741,7 @@ void ThreadRPCServer(void* parg)
         PrintException(&e, "ThreadRPCServer()");
     } catch (...) {
         vnThreadsRunning[THREAD_RPCLISTENER]--;
-        PrintException(NULL, "ThreadRPCServer()");
+        PrintException(nullptr, "ThreadRPCServer()");
     }
     printf("ThreadRPCServer exited\n");
 }
@@ -1483,7 +1483,7 @@ int CommandLineRPC(int argc, char *argv[])
     }
     catch (...)
     {
-        PrintException(NULL, "CommandLineRPC()");
+        PrintException(nullptr, "CommandLineRPC()");
     }
 
     if (strPrint != "")
@@ -1502,18 +1502,18 @@ int main(int argc, char *argv[])
 #ifdef _MSC_VER
     // Turn off Microsoft heap dump noise
     _CrtSetReportMode(_CRT_WARN, _CRTDBG_MODE_FILE);
-    _CrtSetReportFile(_CRT_WARN, CreateFile("NUL", GENERIC_WRITE, 0, NULL, OPEN_EXISTING, 0, 0));
+    _CrtSetReportFile(_CRT_WARN, CreateFile("NUL", GENERIC_WRITE, 0, nullptr, OPEN_EXISTING, 0, 0));
 #endif
-    setbuf(stdin, NULL);
-    setbuf(stdout, NULL);
-    setbuf(stderr, NULL);
+    setbuf(stdin, nullptr);
+    setbuf(stdout, nullptr);
+    setbuf(stderr, nullptr);
 
     try
     {
         if (argc >= 2 && string(argv[1]) == "-server")
         {
             printf("server ready\n");
-            ThreadRPCServer(NULL);
+            ThreadRPCServer(nullptr);
         }
         else
         {
@@ -1523,7 +1523,7 @@ int main(int argc, char *argv[])
     catch (std::exception& e) {
         PrintException(&e, "main()");
     } catch (...) {
-        PrintException(NULL, "main()");
+        PrintException(nullptr, "main()");
     }
     return 0;
 }