rafthttp: build transport inside pkg instead of passed-in

rafthttp has different requirements for connections created by the
transport for different usage, and this is hard to achieve when giving
one http.RoundTripper. Pass into pkg the data needed to build transport
now, and let rafthttp build its own transports.

etcdserver/server_test.go

@@ -1468,7 +1468,7 @@ func (n *readyNode) Ready() <-chan raft.Ready { return n.readyc }
 
 type nopTransporter struct{}
 
-func (s *nopTransporter) Start()                              {}
+func (s *nopTransporter) Start() error                        { return nil }
 func (s *nopTransporter) Handler() http.Handler               { return nil }
 func (s *nopTransporter) Send(m []raftpb.Message)             {}
 func (s *nopTransporter) AddRemote(id types.ID, us []string)  {}