Make nTimeConnected const in CNode

src/net.h

@@ -587,7 +587,7 @@ public:
 
     std::atomic<int64_t> nLastSend;
     std::atomic<int64_t> nLastRecv;
-    int64_t nTimeConnected;
+    const int64_t nTimeConnected;
     std::atomic<int64_t> nTimeOffset;
     const CAddress addr;
     std::string addrName;