introduce a separate code paths for Short Header packet handling

[marten-seemann]

Part of #3526.

This allows us to avoid the expensive code path of first parsing a wire.Header, then converting it into a wire.ExtendedHeader, which then has most of its field unset, since it can represent the entire complexity of a Long Header.

This change brings down the allocations for receiving data significantly.
In follow-up PRs, I'm planning to:

• get rid of the wire.ShortHeader struct. The 4 fields could be return values, making sure that all allocations happen on the stack
• improve the code path for sending short header packets

[codecov]

Codecov Report

Base: 85.56% // Head: 85.47% // Decreases project coverage by -0.08% ⚠️

Coverage data is based on head (4f3d3b3) compared to base (3f1adfd).
Patch coverage: 84.58% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3534      +/-   ##
==========================================
- Coverage   85.56%   85.47%   -0.08%     
==========================================
  Files         137      138       +1     
  Lines       10087    10221     +134     
==========================================
+ Hits         8630     8736     +106     
- Misses       1080     1102      +22     
- Partials      377      383       +6     

Impacted Files	Coverage Δ
qlog/event.go	98.11% <ø> (ø)
connection.go	76.85% <78.12%> (-0.80%)	⬇️
internal/wire/short_header.go	83.78% <83.78%> (ø)
packet_unpacker.go	86.73% <93.02%> (+0.70%)	⬆️
internal/utils/byteorder_big_endian.go	85.00% <100.00%> (+1.67%)	⬆️
internal/wire/header.go	87.88% <100.00%> (+0.15%)	⬆️
logging/multiplex.go	95.45% <100.00%> (+0.11%)	⬆️
qlog/packet_header.go	96.77% <100.00%> (+0.62%)	⬆️
qlog/qlog.go	91.13% <100.00%> (+0.35%)	⬆️
server.go	79.59% <100.00%> (ø)
... and 2 more

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.