cannot call shutdown from within action server's handle goal callback

Currently, if you do something like this in the handle goal callback for an Action server:

RCLCPP_ERROR(logger, "there was a problem");
rclcpp::shutdown();
return rclcpp_action::GoalResponse::REJECT;

Then you'll get an error during shutdown like this:

libc++abi.dylib: terminating with uncaught exception of type rclcpp::exceptions::RCLError: feedback publisher is invalid, at /Users/william/ros2_ws/src/ros2/rcl/rcl_action/src/rcl_action/action_server.c:913

This is because the action server tries to send a rejection response after shutdown is called (and therefore after the context is invalid).

This is actually a symptom of a larger issue which is that some communication activity needs to continue to work after shutdown. So if we address that issue, then this one may be resolved directly.

Start by reproducing the shutdown call from an action server handle goal callback, then inspect the rejection-response path in rcl_action/action_server.c around the reported failure. Done means shutdown from the callback no longer causes an uncaught RCLError and the action server's post-shutdown communication behavior is defined.