disable fluentd time related file-splitting functionality

Check [CONTRIBUTING guideline](https://github.com/fluent/fluentd/blob/master/CONTRIBUTING.md) first and here is the list to help us investigate the problem.

**Is your feature request related to a problem? Please describe.**

I use out file plugin in my case, the out file is splited daily by default.
but I would like to disable fluentd time related file-splitting functionality so that I can use logrotate to fully split file based on time and size.

I don't know how to disable fluentd time related file-splitting functionality.

hope it can be supported.

I'm looking forward to your feedback:)