Code modernization: subfolders (#6283)

* Code modernization: subfolders

Modernize various code on subfolders client, network, script, threading, unittests, util

* empty function
* default constructor/destructor
* for range-based loops
* use emplace_back instead of push_back
* C++ STL header style
* Make connection.cpp readable in a pointed place + typo

src/script/lua_api/l_object.h

@@ -35,7 +35,7 @@ class ObjectRef : public ModApiBase {
 public:
 	ObjectRef(ServerActiveObject *object);
 
-	~ObjectRef();
+	~ObjectRef() = default;
 
 	// Creates an ObjectRef and leaves it on top of stack
 	// Not callable from Lua; all references are created on the C side.