nsq_to_file: make the interval at which we check for log rotation configurable

[alpaker]

Currently nsq_to_file checks whether to rotate the output file every 30s, a hard-coded interval. It would be convenient if this quantity were customizable.

[ploxiln]

We don't require a separate issue for every pull request, you can propose a feature or a bugfix in a lone PR. Closing in favor of #1071