TQueue: provide peekAll

I'm missing a `peekAll` on `TQueue`. 
Background: I have a queue and I want to implement a debugging endpoint in my app where I can inspect the queue. 
Currently I have to resort to this workaround/hack:

```scala
(for {
    allItems <- skippedItemsQueue.takeAll
    _ <- skippedItemsQueue.offerAll(allItems)
  } yield allItems).commit
```

because there is only a `peek` and a `peekOption`. 