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_storage.h

@@ -50,7 +50,7 @@ private:
 
 public:
 	StorageRef(ModMetadata *object);
-	~StorageRef() {}
+	~StorageRef() = default;
 
 	static void Register(lua_State *L);
 	static void create(lua_State *L, ModMetadata *object);