examples: Add longpoll server and client

This is an example of a race-free long-poll server and client. It uses a
redirection method to signal that the "Watch" is running.

Other race-free methods exist.

examples/longpoll/redirect-client.go

@@ -0,0 +1,52 @@
+// This is an example longpoll client. The connection to the corresponding
+// server initiates a request on a "Watch". It then waits until a redirect is
+// received from the server which indicates that the watch is ready. To signal
+// than an event on this watch has occurred, the server sends a final message.
+package main
+
+import (
+	"bytes"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"math/rand"
+	"net/http"
+	"time"
+)
+
+const (
+	timeout = 15
+)
+
+func main() {
+	log.Printf("Starting...")
+
+	checkRedirectFunc := func(req *http.Request, via []*http.Request) error {
+		log.Printf("Watch is ready!")
+		return nil
+	}
+
+	client := &http.Client{
+		Timeout:       time.Duration(timeout) * time.Second,
+		CheckRedirect: checkRedirectFunc,
+	}
+
+	id := rand.Intn(2 ^ 32 - 1)
+	body := bytes.NewBufferString("hello")
+	url := fmt.Sprintf("http://127.0.0.1:12345/watch?id=%d", id)
+	req, err := http.NewRequest("GET", url, body)
+	if err != nil {
+		log.Printf("err: %+v", err)
+		return
+	}
+	result, err := client.Do(req)
+	if err != nil {
+		log.Printf("err: %+v", err)
+		return
+	}
+	log.Printf("Event received: %+v", result)
+
+	s, err := ioutil.ReadAll(result.Body) // TODO: apparently we can stream
+	result.Body.Close()
+	log.Printf("Response: %+v", string(s))
+}