Round initialTime in RollingFileManager

[kelunik]

Caching of filesystem timestamps (at least in ext4 on Linux) results in non-accurate creation timestamps.

Thus, let's round it to the second.

Rounding is also applied for the lastModified time, but I'm not sure whether it's needed there.

Fixes #3068.

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[vy]

While @ppkarwasz is the main figure stewarding #3068 and this one, I'd appreciate one or more new tests and a changelog entry.

[kelunik]

@vy If you have an idea on how to test this properly, I'm happy to write a test.

[github-actions]

Job	Requested goals	Build Tool Version	Build Outcome	Build Scan®
build-macos-latest	clean install	3.9.8	✅
build-ubuntu-latest	clean install	3.9.8	✅
build-windows-latest	clean install	3.9.8	✅

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[vy]

If you have an idea on how to test this properly, I'm happy to write a test.

@kelunik, create a package-private initialFileTime(File,FileTime) that initialFileTime(File) delegates to, and write a unit test against initialFileTime(File,FileTime) verifying that the sub-second precision FileTime arguments are rounded as expected?