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_nodemeta.cpp

@@ -182,10 +182,6 @@ NodeMetaRef::NodeMetaRef(Metadata *meta):
 {
 }
 
-NodeMetaRef::~NodeMetaRef()
-{
-}
-
 // Creates an NodeMetaRef and leaves it on top of stack
 // Not callable from Lua; all references are created on the C side.
 void NodeMetaRef::create(lua_State *L, v3s16 p, ServerEnvironment *env)