avoid allocating when detecting lost packets

[marten-seemann]

Admittedly, avoiding the allocation for detectAndRemoveAckedPackets would be more important, but it's also more difficult, so let's start with the low hanging fruits.

[codecov]

Codecov Report

Merging #2898 (85fd5a2) into master (69158cf) will increase coverage by 0.02%.
The diff coverage is 58.33%.

@@            Coverage Diff             @@
##           master    #2898      +/-   ##
==========================================
+ Coverage   85.77%   85.80%   +0.02%     
==========================================
  Files         133      133              
  Lines        9187     9174      -13     
==========================================
- Hits         7880     7871       -9     
+ Misses        959      955       -4     
  Partials      348      348              

Impacted Files	Coverage Δ
internal/flowcontrol/base_flow_controller.go	100.00% <ø> (ø)
internal/ackhandler/sent_packet_handler.go	73.56% <45.95%> (+0.51%)	⬆️
internal/flowcontrol/connection_flow_controller.go	100.00% <100.00%> (ø)
internal/flowcontrol/stream_flow_controller.go	100.00% <100.00%> (ø)

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