Make objects in range declarations immutable by default. Avoid unnecessary copying of objects in range declarations.

src/test/test_bitcoin.cpp

@@ -29,7 +29,7 @@ void CConnmanTest::AddNode(CNode& node)
 void CConnmanTest::ClearNodes()
 {
     LOCK(g_connman->cs_vNodes);
-    for (CNode* node : g_connman->vNodes) {
+    for (const CNode* node : g_connman->vNodes) {
         delete node;
     }
     g_connman->vNodes.clear();