WorkerThreadPool: Avoid most runtime allocations

Just a little optimization.

**NOTE:**
With `RID_Owner` we could replace each pair of `PagedAllocator` and
`HashMap`-of-ids-to-pointers. However, that would force us to expose
`RID` as the task/group id, instead of `int`, which would break the
API. Too bad. Let's wait until Godot 5.0.

core/object/worker_thread_pool.cpp

@@ -613,13 +613,14 @@ void WorkerThreadPool::finish() {
 		return;
 	}
 
-	task_mutex.lock();
-	SelfList<Task> *E = low_priority_task_queue.first();
-	while (E) {
-		print_error("Task waiting was never re-claimed: " + E->self()->description);
-		E = E->next();
+	{
+		MutexLock lock(task_mutex);
+		SelfList<Task> *E = low_priority_task_queue.first();
+		while (E) {
+			print_error("Task waiting was never re-claimed: " + E->self()->description);
+			E = E->next();
+		}
 	}
-	task_mutex.unlock();
 
 	{
 		MutexLock lock(task_mutex);
@@ -632,6 +633,13 @@ void WorkerThreadPool::finish() {
 		data.thread.wait_to_finish();
 	}
 
+	{
+		MutexLock lock(task_mutex);
+		for (KeyValue<TaskID, Task *> &E : tasks) {
+			task_allocator.free(E.value);
+		}
+	}
+
 	threads.clear();
 }