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/util/srp.cpp

@@ -31,13 +31,14 @@
 	#include <windows.h>
 	#include <wincrypt.h>
 #else
-	#include <time.h>
+	#include <ctime>
+
 #endif
 // clang-format on
 
-#include <stdlib.h>
-#include <string.h>
-#include <stdio.h>
+#include <cstdlib>
+#include <cstring>
+#include <cstdio>
 
 #include <config.h>