flow-monitor (fixes #1097): add logging of min. and max. packet delay in FlowMonitor

FYI: Re-submission of MR !2011 (closed), since I have messed up the original branch.

This merge request adds the logging of the maximum and minimum packet delay of any received packet via FlowMonitor.

The values are stored in FlowMonitor::FlowStats::maxDelay / minDelay, analog to the other values reported there.

A quick style check via utils/check-style-clang-format.py-format.py src/flow-monitor (clang-format v15, ubuntu 20.04) shows no remaining issues.

UPDATE: this new MR also contains the minimum packet delay, is rebased to include the XML formatting changes of !2012 (merged) and reverses the variable order in the delay update logic (@edalm's nitpick)