use the receipt of a Retry packet to get a first RTT estimate

[marten-seemann]

Fixes #2178.

[codecov]

Codecov Report

Merging #2588 into master will decrease coverage by 0.03%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #2588      +/-   ##
==========================================
- Coverage   86.27%   86.25%   -0.03%     
==========================================
  Files         122      122              
  Lines        9755     9764       +9     
==========================================
+ Hits         8416     8421       +5     
- Misses        997      999       +2     
- Partials      342      344       +2     

Impacted Files	Coverage Δ
internal/ackhandler/sent_packet_handler.go	73.39% <60.00%> (-0.35%)	⬇️

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