Improve QPS calculation

[DL6ER]

What does this implement/fix?

Outsource counter resetting into a dedicated function periodically run by the GC thread to make the average computation independent from external trigger sources. This solves the problem of artificially high QPS values when no queries are processed by FTL.

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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[yubiuser]

It seems the counter is not updated every second. It stays the same sometimes for a longer time but suddenly starts changing every second and then a longer interval occurs again.