Barrier node does not flush last window with groupBy

In the following streaming task, the barrier node will not flush the last window of data through the pipeline:

```
stream
|from()
.measurement(measurement)
|groupBy(tags)
|barrier()
.idle(30s)
|window()
.period(1m)
.every(1m)
.align()
```

However if we replace the `|groupBy` node with the `.groupBy` property on the `|from` node like so:

```
stream
|from()
.measurement(measurement)
.groupBy(tags)
|barrier()
.idle(30s)
|window()
.period(1m)
.every(1m)
.align()
```

the barrier node does indeed flush the last window through the pipeline.

Reproduce the streaming task shown in the issue, first with groupBy as its own node and then with groupBy on from(). Compare how barrier() and window() handle the final window in each case. Done means the standalone groupBy form flushes the last window through the pipeline consistently with the from().groupBy form, with a regression test covering both cases.

Written by the indexing model from the issue text.