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 
OK. We might want to solve these kind of aesthetics later. Current priority is to keep raft pkg itself as simple and clean as possible(it might add complexity to application). After we stabilize the functionality/interface/testing of the pkg, we will start to care more about the aesthetics problem. Reasonable?