Add EXIT statement.

This pull request adds the `EXIT` statement to SkyQL. When the `EXIT` is reached then the query is immediately halted for the current object and it moves on to the next one.

Example:

```
DECLARE event_count AS INTEGER
SET event_count = event_count + 1
SELECT count() AS count
WHEN event_count > 10 THEN
    EXIT
END
```

That's not really the most useful example but it'll count their first 10 events for each object.