The ROS 2 C++ Executors

[alsora]

I propose the following roadmap to take the EventsExecutor out of the experimental namespace:

P.S. note that I also started a discussion dedicated to how the experimental workspace should be managed here: On the `rclcpp::experimental` namespace.

• Fix known bugs:
  • Sim-time not respected: TimersManager doesn't follow ROS time · Issue #2480 · ros2/rclcpp · GitHub (see here a draft proposal from @JM_ROS: )
  • No support for graph guard condition: EventsExecutor doesn't support rcl guard conditions · Issue #2574 · ros2/rclcpp · GitHub

A further promotion to default executor can be discussed as a following step.

@clalancette

• It probably needs to have a multi-threaded version, though this may be negotiable.
• It needs to be run against all of the tests we currently have. Note that for most tests in the core, we use the default (SingleThreadedExecutor) by default, unless the test explicitly requests otherwise.

Once those 3 major things are done, I think the EventsExecutor could be promoted out of the experimental namespace, and then we could consider making it the default.

@clalancette I think these 2 should be requirements to become the default, not to go out of experimental.