Use buffered channel for graceful shutdown

According to https://golang.org/pkg/os/signal/#example_Notify
buffered channel should be used to avoid missing signal.

Author: shihanng

cookbook/graceful-shutdown/server.go

@@ -29,7 +29,7 @@ func main() {
 
 	// Wait for interrupt signal to gracefully shutdown the server with
 	// a timeout of 10 seconds.
-	quit := make(chan os.Signal)
+	quit := make(chan os.Signal, 1)
 	signal.Notify(quit, os.Interrupt)
 	<-quit
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)