don't allocate for acked packets

[marten-seemann]

The problem here is that the congestion controller expects that OnPacketLost is called before OnPacketAcked (or doesn't it?), so we need to somehow create a slice of acknowledged packets. We can just reuse the same slice every time though, thereby effectively avoiding allocations.

[codecov]

Codecov Report

Merging #2899 (8d14d76) into master (69158cf) will increase coverage by 0.01%.
The diff coverage is 88.24%.

@@            Coverage Diff             @@
##           master    #2899      +/-   ##
==========================================
+ Coverage   85.77%   85.79%   +0.01%     
==========================================
  Files         133      133              
  Lines        9187     9181       -6     
==========================================
- Hits         7880     7876       -4     
+ Misses        959      958       -1     
+ Partials      348      347       -1     

Impacted Files	Coverage Δ
internal/flowcontrol/base_flow_controller.go	100.00% <ø> (ø)
internal/ackhandler/sent_packet_handler.go	73.52% <66.67%> (+0.47%)	⬆️
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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