Adapt to upstream changes in etcd/raft.

Node ids are now uint64, and the application is required to send
committed EntryConfChanges back into the raft system.

@cockroachdb/developers 
I don't want to attach any data to a noop; it's just for aesthetics so all the decision-making is handled by `switch entry.Type`, instead of a second-level condition under the `normal` case.