save dummy packets in the packet history when skipping packet numbers

[marten-seemann]

Depends on #2752 and #2750.

By saving a dummy packet in the packet history we can get rid of the separate tracking of skipped packet numbers.

@lucas-clemente Only the last 2 commits are new in this PR.

[codecov]

Codecov Report

Merging #2753 into master will increase coverage by 0.01%.
The diff coverage is 86.21%.

@@            Coverage Diff             @@
##           master    #2753      +/-   ##
==========================================
+ Coverage   86.71%   86.72%   +0.01%     
==========================================
  Files         128      128              
  Lines       10033    10039       +6     
==========================================
+ Hits         8700     8706       +6     
  Misses       1006     1006              
  Partials      327      327              

Impacted Files	Coverage Δ
internal/ackhandler/packet_number_generator.go	100.00% <ø> (ø)
internal/ackhandler/sent_packet_handler.go	74.73% <75.00%> (+0.48%)	⬆️
internal/ackhandler/sent_packet_history.go	96.61% <90.48%> (-3.39%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 74f2610...12c9427. Read the comment docs.