Modernize RWLock

- Based on C++14's `shared_time_mutex`
- No more need to allocate-deallocate or check for null
- No pointer anymore, just a member variable
- Platform-specific implementations no longer needed
- Simpler for `NO_THREADS`

main/main.cpp

@@ -388,8 +388,6 @@ Error Main::test_setup() {
 
 	engine = memnew(Engine);
 
-	ClassDB::init();
-
 	register_core_types();
 	register_core_driver_types();
 
@@ -507,8 +505,6 @@ Error Main::setup(const char *execpath, int argc, char *argv[], bool p_second_ph
 
 	engine = memnew(Engine);
 
-	ClassDB::init();
-
 	MAIN_PRINT("Main: Initialize CORE");
 
 	register_core_types();