Today, `stream.Stream` is structured as:
```go
case <-streamer.Done():
  // call Close()
case <-time.After(fetchDelay):
  // call Fetch()
```
The problem is that it's possible that the `Done()` event is triggered at
the same time as `time.After(fetchDelay)` and in that scenario the
behavior is undeterministic.
It's possible that we call `Fetch` a second time. For the ECS deployment
streamer, this means that we would previous close the `s.done` channel
twice. Wrapping the call with a `sync.Once` removes this concern.


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