refactor RTT measurements to simplify the sentPacketHistory

[marten-seemann]

No description provided.

[codecov]

Codecov Report

Merging #2747 into master will increase coverage by 0.01%.
The diff coverage is 63.64%.

@@            Coverage Diff             @@
##           master    #2747      +/-   ##
==========================================
+ Coverage   86.52%   86.53%   +0.01%     
==========================================
  Files         128      128              
  Lines        9960     9955       -5     
==========================================
- Hits         8617     8614       -3     
+ Misses       1010     1009       -1     
+ Partials      333      332       -1     

Impacted Files	Coverage Δ
internal/ackhandler/sent_packet_history.go	100.00% <ø> (ø)
internal/ackhandler/sent_packet_handler.go	73.48% <63.64%> (+0.38%)	⬆️

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