ResourceLoader: Enhance deadlock prevention

Benefits:
- Simpler code. The main load function is renamed so it's apparent that it's not just a thread entry point anymore.
- Cache and thread modes of the original task are honored. A beautiful consequence of this is that, unlike formerly, re-issued loads can use the resource cache, which makes this mechanism much more performant.
- The newly added getter for caller task id in WorkerThreadPool allows to remove the custom tracking of that in ResourceLoader.
- The check to replace a cached resource and the replacement itself happen atomically. That fixes deadlock prevention leading to multiple resource instances of the same one on disk. As a side effect, it also makes the regular check for replace load mode more robust.

(cherry picked from commit 28619e26cf)

core/object/worker_thread_pool.cpp

@@ -665,6 +665,15 @@ int WorkerThreadPool::get_thread_index() {
 	return singleton->thread_ids.has(tid) ? singleton->thread_ids[tid] : -1;
 }
 
+WorkerThreadPool::TaskID WorkerThreadPool::get_caller_task_id() {
+	int th_index = get_thread_index();
+	if (th_index != -1 && singleton->threads[th_index].current_task) {
+		return singleton->threads[th_index].current_task->self;
+	} else {
+		return INVALID_TASK_ID;
+	}
+}
+
 #ifdef THREADS_ENABLED
 uint32_t WorkerThreadPool::thread_enter_unlock_allowance_zone(Mutex *p_mutex) {
 	return _thread_enter_unlock_allowance_zone(p_mutex, false);