delete unacknowledged packets from the packet history after 3 PTOs

[marten-seemann]

This is a fairly big change how we keep track of sent packets. We used to evict packets from the history once they got acknowledged or declared lost.
With this change, packets are only evicted when they are acknowledged. When a packet is declared lost, we still keep it in the history. This allows us generate RTT measurements for packets that were spuriously declared lost.
Packets are now deleted from the history after 3 PTOs (and only if they have been declared lost before).

[codecov]

Codecov Report

Merging #2750 into master will increase coverage by 0.03%.
The diff coverage is 92.16%.

@@            Coverage Diff             @@
##           master    #2750      +/-   ##
==========================================
+ Coverage   86.69%   86.71%   +0.03%     
==========================================
  Files         128      128              
  Lines       10013    10033      +20     
==========================================
+ Hits         8680     8700      +20     
  Misses       1006     1006              
  Partials      327      327              

Impacted Files	Coverage Δ
internal/ackhandler/sent_packet_handler.go	74.25% <87.50%> (+0.34%)	⬆️
internal/ackhandler/sent_packet_history.go	100.00% <100.00%> (ø)

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