use a sync.Pool for ACK frames

[marten-seemann]

Closes #2885.

[codecov]

Codecov Report

Base: 85.50% // Head: 85.41% // Decreases project coverage by -0.09% ⚠️

Coverage data is based on head (63764c4) compared to base (62b8278).
Patch coverage: 66.67% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3547      +/-   ##
==========================================
- Coverage   85.50%   85.41%   -0.09%     
==========================================
  Files         138      139       +1     
  Lines       10241    10266      +25     
==========================================
+ Hits         8756     8768      +12     
- Misses       1102     1114      +12     
- Partials      383      384       +1     

Impacted Files	Coverage Δ
internal/logutils/frame.go	59.26% <0.00%> (-40.74%)	⬇️
connection.go	77.00% <60.00%> (-0.09%)	⬇️
internal/ackhandler/received_packet_history.go	96.43% <100.00%> (-0.24%)	⬇️
internal/ackhandler/received_packet_tracker.go	86.21% <100.00%> (+0.32%)	⬆️
internal/wire/ack_frame.go	94.78% <100.00%> (ø)
internal/wire/ack_frame_pool.go	100.00% <100.00%> (ø)

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