When shadow_fd structs _that are owned by known protocol objects_
have their reference count drop to zero, they are deleted.

Newly created shadow_fd's begin with a reference count of 1,
which is reduced when the translated fd to sent to the
display/application/other waypipe socket end. This ensures
that the object is not deleted early, even if the compositor
e.g. wl_display.delete_id's the fd owning object before receiving
it.