Etcd is a distributed, highly available name-value store that is
the basis of many clustered applications. However, it can also be
run on a standalone machine. Using etcd allows live monitoring
and possibly changing configuration while the sim is running.
This is not used for anything - appearances are always properties of objects with ids (ScenePresence, AgentCircuitData) and just has the potential to get out of sync when the appearance is cloned.