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.

chasquid.go

@@ -103,6 +103,8 @@ func main() {
 	// as a remote domain (for loops, alias resolutions, etc.).
 	s.AddDomain("localhost")
 
+	s.LoadQueue(conf.DataDir + "/queue")
+
 	// Load the addresses and listeners.
 	systemdLs, err := systemd.Listeners()
 	if err != nil {
@@ -249,6 +251,15 @@ func (s *Server) AddUserDB(domain string, db *userdb.DB) {
 	s.userDBs[domain] = db
 }
 
+func (s *Server) LoadQueue(path string) {
+	q := queue.New(path, s.localDomains)
+	err := q.Load()
+	if err != nil {
+		glog.Fatalf("Error loading queue: %v", err)
+	}
+	s.queue = q
+}
+
 func (s *Server) getTLSConfig() (*tls.Config, error) {
 	var err error
 	conf := &tls.Config{}
@@ -275,11 +286,6 @@ func (s *Server) ListenAndServe() {
 		glog.Fatalf("Error loading TLS config: %v", err)
 	}
 
-	// TODO: Create the queue when creating the server?
-	// Or even before, and just give it to the server?
-	s.queue = queue.New(
-		&courier.Procmail{}, &courier.SMTP{}, s.localDomains)
-
 	for m, addrs := range s.addrs {
 		for _, addr := range addrs {
 			// Listen.