Honor the user holding onto shared_ptrs during subscription callbacks

While debugging https://github.com/ros2/rmw_cyclonedds/issues/469 , we found that loaned messages do not properly honor the user holding onto shared_ptrs. In particular, if you look at the code in https://github.com/ros2/rclcpp/blob/126d517193b6df8177680898e23906a85792eaf6/rclcpp/src/rclcpp/executor.cpp#L626-L662 , you can see that the executor always immediately returns the loan to the rmw layer, regardless of whether the user took a shared_ptr reference.

We have worked around this for now by disabling loans by default, but that is not an ideal solution. Instead, what we should do is make a custom destructor for the shared_ptr that we create, which is responsible for returning the loan to the rmw layer. With that in place, we could turn loans back on, and still be safe when users take shared_ptr references.

Start in rclcpp/src/rclcpp/executor.cpp at lines 626-662 and trace how loaned messages are returned to the rmw layer during subscription callbacks. Verify the shared_ptr lifetime behavior and define done as returning the loan only after the user's final shared_ptr reference is released, while keeping loaned-message use safe.