Find replacement for axros in ROS 2

[cbrxyz]

As we migrate to ROS 2 (uf-mil/mil#1124), we will need to find a suitable replacement for asynchronous operations in ROS 2. There are a few ways we could go about this:

1. We could re-implement the features of axros in ROS 2, and barely change the public API of this library.
2. We can use some of the partial-async compatible features in rclpy rather than continuing development of axros
3. We can try to convert as much of our code using asyncio to use threads instead
4. We could develop async wrappers around rclpy and publish those as part of axros2

The direction of this library will have to be determined with time. Relevant issues:

• Runtime error when a long async callback is triggered ros2/rclpy#962
• Executor cannot handle asyncio.sleep() in coroutine ros2/rclpy#279
• async wait/sleep implementation ros2/rclpy#1234

Most of these issues just contain workarounds to make asyncio work with the existing implementation of rclpy.