queue: Calculate next delay based on creation time

Calculating the next delay based on the previous delay causes daemon
restarts to start from scratch, as we don't persist it.

This can cause a few server restarts to generate many unnecessary sends.

This patch changes the next delay calculation to use the creation time
instead, and also adds a <=1m random perturbation to avoid all queued
emails to be retried at the exact same time after a restart.

internal/queue/queue_test.go

@@ -263,3 +263,25 @@ func TestPipes(t *testing.T) {
 		t.Errorf("pipe delivery failed: %v", err)
 	}
 }
+
+func TestNextDelay(t *testing.T) {
+	cases := []struct{ since, min time.Duration }{
+		{10 * time.Second, 1 * time.Minute},
+		{3 * time.Minute, 5 * time.Minute},
+		{7 * time.Minute, 10 * time.Minute},
+		{15 * time.Minute, 20 * time.Minute},
+		{30 * time.Minute, 20 * time.Minute},
+	}
+	for _, c := range cases {
+		// Repeat each case a few times to exercise the perturbation a bit.
+		for i := 0; i < 10; i++ {
+			delay := nextDelay(time.Now().Add(-c.since))
+
+			max := c.min + 1*time.Minute
+			if delay < c.min || delay > max {
+				t.Errorf("since:%v  expected [%v, %v], got %v",
+					c.since, c.min, max, delay)
+			}
+		}
+	}
+}