Add a skeleton queue

This patch introduces a basic, in-memory queue that only holds emails for now.

This slows down the benchmarks because we don't yet have a way to wait for
delivery (even if fake), that will come in later patches.

internal/queue/queue_test.go

@@ -0,0 +1,70 @@
+package queue
+
+import (
+	"bytes"
+	"testing"
+	"time"
+)
+
+func TestBasic(t *testing.T) {
+	q := New()
+
+	id, err := q.Put("from", []string{"to"}, []byte("data"))
+	if err != nil {
+		t.Fatalf("Put: %v", err)
+	}
+
+	if len(id) < 6 {
+		t.Errorf("short ID: %v", id)
+	}
+
+	q.mu.RLock()
+	item := q.q[id]
+	q.mu.RUnlock()
+
+	// TODO: There's a race because the item may finish the loop before we
+	// poll it from the queue, and we would get a nil item in that case.
+	// We have to live with this for now, and will close it later once we
+	// implement deliveries.
+	if item == nil {
+		t.Logf("hit item race, nothing else to do")
+		return
+	}
+
+	if item.From != "from" || item.To[0] != "to" ||
+		!bytes.Equal(item.Data, []byte("data")) {
+		t.Errorf("different item: %#v", item)
+	}
+}
+
+func TestFullQueue(t *testing.T) {
+	q := New()
+
+	// Force-insert maxQueueSize items in the queue.
+	oneID := ""
+	for i := 0; i < maxQueueSize; i++ {
+		item := &Item{
+			ID:      <-newID,
+			From:    "from",
+			To:      []string{"to"},
+			Data:    []byte("data"),
+			Created: time.Now(),
+			Results: map[string]error{},
+		}
+		q.q[item.ID] = item
+		oneID = item.ID
+	}
+
+	// This one should fail due to the queue being too big.
+	id, err := q.Put("from", []string{"to"}, []byte("data"))
+	if err != queueFullError {
+		t.Errorf("Not failed as expected: %v - %v", id, err)
+	}
+
+	// Remove one, and try again: it should succeed.
+	q.Remove(oneID)
+	_, err = q.Put("from", []string{"to"}, []byte("data"))
+	if err != nil {
+		t.Errorf("Put: %v", err)
+	}
+}