Method wait_pop

BlockingQueue.h:21–27

		return true;
	}
	ITEM wait_pop() { // blocking pop
		std::unique_lock<Mutex> lock(mutex);
		condition.wait(lock, [this] {return !queue.empty(); });
		ITEM const value = std::move(queue.front());
		queue.pop();
		return value;
	}
	bool empty() const { // queue is empty?
		std::lock_guard<Mutex> lock(mutex);
		return queue.empty();