make it possible to trace acknowledged packets

[marten-seemann]

We need this to get an estimate of the packet loss rate: lost / (acked + lost).

[codecov]

Codecov Report

Merging #3134 (c4073fb) into master (e098ccd) will decrease coverage by 0.03%.
The diff coverage is 54.55%.

@@            Coverage Diff             @@
##           master    #3134      +/-   ##
==========================================
- Coverage   85.47%   85.44%   -0.03%     
==========================================
  Files         131      131              
  Lines        9650     9653       +3     
==========================================
  Hits         8248     8248              
- Misses       1034     1036       +2     
- Partials      368      369       +1     

Impacted Files	Coverage Δ
qlog/qlog.go	90.64% <0.00%> (-0.34%)	⬇️
internal/ackhandler/sent_packet_handler.go	77.25% <55.56%> (-0.44%)	⬇️
logging/multiplex.go	97.56% <100.00%> (+0.06%)	⬆️

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