http3: reduce usage of bytes.Buffer

[marten-seemann]

Part of #3526.

[codecov]

Codecov Report

Base: 85.49% // Head: 85.50% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (6fb0a65) compared to base (dfd35cb).
Patch coverage: 88.89% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3539      +/-   ##
==========================================
+ Coverage   85.49%   85.50%   +0.01%     
==========================================
  Files         138      138              
  Lines       10237    10241       +4     
==========================================
+ Hits         8752     8756       +4     
  Misses       1102     1102              
  Partials      383      383              

Impacted Files	Coverage Δ
http3/request_writer.go	63.39% <66.67%> (-0.32%)	⬇️
http3/response_writer.go	76.09% <71.43%> (+0.53%)	⬆️
http3/http_stream.go	93.55% <85.71%> (+0.69%)	⬆️
http3/client.go	90.41% <100.00%> (ø)
http3/frames.go	84.81% <100.00%> (+0.19%)	⬆️
http3/server.go	74.29% <100.00%> (ø)

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