Node cleanup

[rjoomen]

These commits make the usage and naming of nodes and internal nodes more consistent. Also, executors and callback groups are simplified to SingleThreaded and MutuallyExclusive where possible (the MultiThreadedExecutor has lower performance).

[marrts]

This looks good to me and is working, however when I first tested it I ran into an issue where my environment wasn't updating because I was using the DEFAULT_JOINT_STATES_TOPIC param from here which absorbs the namespace of the node. I feel like the default topic should be /joint_states to avoid getting namespaced and more predictable, consistent behavior.