simplify packing of ACK-only packets

[marten-seemann]

Depends on #3544.

[codecov]

Codecov Report

Base: 85.50% // Head: 85.59% // Increases project coverage by +0.10% 🎉

Coverage data is based on head (818768d) compared to base (c57ea6f).
Patch coverage: 82.93% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3545      +/-   ##
==========================================
+ Coverage   85.50%   85.59%   +0.10%     
==========================================
  Files         138      138              
  Lines       10254    10218      -36     
==========================================
- Hits         8767     8746      -21     
+ Misses       1103     1091      -12     
+ Partials      384      381       -3     

Impacted Files	Coverage Δ
connection.go	76.53% <50.00%> (-0.56%)	⬇️
packet_packer.go	87.77% <100.00%> (+3.61%)	⬆️

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