queue: Implement persistency

This patch makes the queue read and write items to disk.

It uses protobuf for serialization. We serialize to text format to make
manual troubleshooting easier, as the performance difference is not very
relevant for us.

test/t-03-queue_persistency/addtoqueue.go

@@ -0,0 +1,52 @@
+// addtoqueue is a test helper which adds a queue item directly to the queue
+// directory, behind chasquid's back.
+//
+// Note that chasquid does NOT support this, we do it before starting up the
+// daemon for testing purposes only.
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"time"
+
+	"blitiri.com.ar/go/chasquid/internal/queue"
+)
+
+var (
+	queueDir = flag.String("queue_dir", ".queue", "queue directory")
+	id       = flag.String("id", "mid1234", "Message ID")
+	from     = flag.String("from", "from", "Mail from")
+	rcpt     = flag.String("rcpt", "rcpt", "Rcpt to")
+)
+
+func main() {
+	flag.Parse()
+
+	data, err := ioutil.ReadAll(os.Stdin)
+	if err != nil {
+		fmt.Printf("error reading data: %v\n", err)
+		os.Exit(1)
+	}
+
+	item := &queue.Item{
+		Message: queue.Message{
+			ID:   *id,
+			From: *from,
+			Rcpt: []*queue.Recipient{
+				{*rcpt, queue.Recipient_EMAIL, queue.Recipient_PENDING},
+			},
+			Data: data,
+		},
+		CreatedAt: time.Now(),
+	}
+
+	os.MkdirAll(*queueDir, 0700)
+	err = item.WriteTo(*queueDir)
+	if err != nil {
+		fmt.Printf("error writing item: %v\n", err)
+		os.Exit(1)
+	}
+}