rafthttp: add AddRemote

Add remotes to rafthttp, who help newly joined members catch up the
progress of the cluster. It supports basic message sending to remote, and
has no stream connection for simplicity. remotes will not be used
after the latest peers have been added into rafthttp.

etcdserver/server_test.go

@@ -1366,6 +1366,7 @@ type nopTransporter struct{}
 
 func (s *nopTransporter) Handler() http.Handler               { return nil }
 func (s *nopTransporter) Send(m []raftpb.Message)             {}
+func (s *nopTransporter) AddRemote(id types.ID, us []string)  {}
 func (s *nopTransporter) AddPeer(id types.ID, us []string)    {}
 func (s *nopTransporter) RemovePeer(id types.ID)              {}
 func (s *nopTransporter) RemoveAllPeers()                     {}