Implement couriers

This patch introduces the couriers, which the queue uses to deliver mail.

We have a local courier (using procmail), a remote courier (uses SMTP), and a
router courier that decides which of the two to use based on a list of local
domains.

There are still a few things pending, but they all have their basic
functionality working and tested.

internal/courier/courier_test.go

@@ -0,0 +1,44 @@
+package courier
+
+import "testing"
+
+// Counter courier, for testing purposes.
+type counter struct {
+	c int
+}
+
+func (c *counter) Deliver(from string, to string, data []byte) error {
+	c.c++
+	return nil
+}
+
+func TestRouter(t *testing.T) {
+	localC := &counter{}
+	remoteC := &counter{}
+	r := Router{
+		Local:  localC,
+		Remote: remoteC,
+		LocalDomains: map[string]bool{
+			"local1": true,
+			"local2": true,
+		},
+	}
+
+	for domain, c := range map[string]int{
+		"local1": 1,
+		"local2": 2,
+		"remote": 9,
+	} {
+		for i := 0; i < c; i++ {
+			r.Deliver("from", "a@"+domain, nil)
+		}
+	}
+
+	if localC.c != 3 {
+		t.Errorf("local mis-count: expected 3, got %d", localC.c)
+	}
+
+	if remoteC.c != 9 {
+		t.Errorf("remote mis-count: expected 9, got %d", remoteC.c)
+	}
+}