rafthttp: always cancel in-flight request when stop streamReader

This problem is totally fixed at 1.5.

go1.5 adds a Request.Cancel channel, which allows for "race free"
cancellation
(8b4278ffb7).
Our implementation relies on it to always cancel in-flight request.

pkg/httputil/cancelreq.go

@@ -0,0 +1,20 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// borrowed from golang/net/context/ctxhttp/cancelreq.go
+
+// +build go1.5
+
+package httputil
+
+import "net/http"
+
+func RequestCanceler(rt http.RoundTripper, req *http.Request) func() {
+	ch := make(chan struct{})
+	req.Cancel = ch
+
+	return func() {
+		close(ch)
+	}
+}

pkg/httputil/cancelreq_go14.go

@@ -0,0 +1,25 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// borrowed from golang/net/context/ctxhttp/cancelreq_go14.go
+
+// +build !go1.5
+
+package httputil
+
+import "net/http"
+
+type requestCanceler interface {
+	CancelRequest(req *http.Request)
+}
+
+func RequestCanceler(rt http.RoundTripper, req *http.Request) func() {
+	c, ok := rt.(requestCanceler)
+	if !ok {
+		return func() {}
+	}
+	return func() {
+		c.CancelRequest(req)
+	}
+}