Flush expired lines before polling input at file source line aggregation

[MOZGIII]

Something that came up during #1852 review. See #1852 (comment).

We should flush expired before polling inner, otherwise, we don't provide proper backpressure and the expired lines may never even flush at all if the load is super-high.

There should be many more tests for the timeout and expiration logic, and once those are done - we can safely alter details like this.