wake up spin when new waitable things are added to a node

[wjwwood]

For example:

auto node = rclcpp::Node::make_shared("node_name");
rclcpp::executors::SingleThreadedExecutor executor;
executor.add_node(node);
std::thread spin_thread([&executor]() {executor.spin();});
auto timer = node->create_timer([]() {printf("hello world\n");});
spin_thread.join();

This code may never print "hello world" if the spin call starts before the timer is created. So to fix this, any time something that is affected by spinning is added to a node, spin should be woken up so that it can consider new work.

To accomplish this, each node will probably need a new guard condition which is triggered any time something new is added that needs to wake up spin. This would probably include subscriptions, timers, service servers or clients, and parameter servers or clients.