Improve load times by skipping serialization of entities when unecessary. #2596

I was investigating gazebosim/gazebo_test_cases#1576 , in my investigation it came to my notice that `sdf::Element` takes forever to destroy (We should open a ticket somewhere about this). If we are skipping serialization we might as well not create and destroy an SDF Element. This hack greatly speeds up the load time for gazebo. Signed-off-by: Arjo Chakravarty <[email protected]>