Fix IAT calculation

[IvanNardi]

We shoudn't reset reference timestamps every 10 packets, i.e IAT
algorithm should be independent from SPLT and BD ones.

[sonarcloud]

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[IvanNardi]

This PR has been marked as Draft since it is only an incomplete POC: all the CI tests will fail.

I am not an expert of the algorithms implemented in nDPI, but I find strange that the packet timestamps used for IAT (Inter Arrival Time) stats are reset every 10 packets (via ndpi_clear_entropy_stats())

Is that correct?

Digging through the GIT history, I found that in the original implementation of the IAT algorithm (9a6f6d9) the timestamps were never reset.
Such a reset has been introduced in 4fd7e57 (see #769), triggering a massive update of all the IAT values in the unit test results.
However, the comment #769 (comment) seems to indicate that such change was unexpected.

@lucaderi: what do you think? That comment was made by you...

I can confirm that with this PR, the new IAT values are identical to the ones before 4fd7e57.

Bottom line: if I am correct I will push a new version updating all the tests; otherwise, sorry for the noise, guys.

[utoni]

Not sure, but IAT seems to me like an extracted feature usable for machine learning.