Use lazy initialization instead of class initialization in NodeId

The class initialization of NodeId triggers a read to a system table. When using the class introduced in CASSANDRA-2911, this happens to be triggered even though this is not necessary. This a a pain because when you use those classes, you don't have to have to set reasonable values for the data_dir and commitlog_dir in cassandra.yaml (it is a pain to have to have a cassandra.yaml in the classpath in the first place, but that's for another ticket).

This ticket proposes using lazy initialization in NodeId to avoid that.