create CallbackSpinner thread in TaskRequestRosApi to avoid manual spinning in scenarios

[rayvburn]

Header:

/// Thread that provides callbacks to be processed (otherwise task requests won't be recognized by ActionServer)
std::thread callback_spinner_;

Source (ctor):

	callback_spinner_ = std::thread([this]() {
		// single-threaded spinning
		ros::spin();
	});

It does not compile ATM:

• ROS Melodic,
• gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0