Another ticker example to handle ticks arriving while the operation is running

[Jonathan-Landeed]

I was experimenting with Ticker and realized this structure could be a useful example.

	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
	defer cancel()
	for range ticker.C {
		if result, err = operation(); err != nil {
			log.Println(err, "will retry...")
			select {
			case <-ctx.Done():
				break
			case <-ticker.C:
				// consume a waiting tick
				continue
			default:
				continue
			}
			break
		}
	}

It ends up being similar to Retry behavior for slow fails, but incrementing the backoff twice for each operation. It's not always useful behavior, but it addresses the "operations that take a while to fail could run in quick succession" problem.

I also experimented with a non-blocking ticker by modifying Ticker.send:

	select {
	case t.c <- tick:
	case <-t.stop:
		return nil
	default:
	}

(also mentioned in #154)
This is more intuitive, as its tick rate is independent of slow fails, but ticks that occur during a slow fail are ignored.

I can make a PR if you're interested.