ot::Message::Ownership enum

This value indicates that the method/function receiving a `Message` instance should take custody of the message (e.g., the method should `Free()` the message if no longer needed).

This value indicates that the method/function receiving a `Message` instance does not own the message (e.g., it should not `Free()` or `Enqueue()` it in a queue). The receiving method/function should create a copy/clone of the message to keep (if/when needed).