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 
We have discussed about it internally. We think it is enough for the application to consume the noop with a nil data. Do you want to attach any data to a noop? Or it is technically meaningless to add a type. But you might want that for aesthetics?