Allow manual rotation of rotating_file_sink

[hjs-ast]

This PR adds the ability to force a rotating_file_sink to rotate its log files on-demand, in addition to the normal size-based rotation.

This is a feature that I implemented as a custom sink for a project where we needed to be able to manually trigger rotation of logs. That implementation effectively required that the entire rotating_file_sink be copy/pasted (as rotating_file_sink is marked as final, and rotate_() is marked as private).

This feels like a generally-useful extension to the standard rotating_file_sink, that I thought it worth offering upstream.

[gabime]

Thanks. I think calling it rotate_files () or rotate() or rotate_now() is clearer though.

[hjs-ast]

I like rotate_now() - I've pushed a change to use that name. It looks like I also broke the pipeline tests on Windows. I think I've also fixed that (but haven't been able to test that locally).

[matteodelseppia]

@gabime just a curiosity. Why rotate_now() here does not require the lock? Is there an implicit lock I'm missing somewhere?

[gabime]

Good catch. A lock is required indeed

[hjs-ast]

I went to fix this, but I see that it's already been done under: #3281

Thanks!