refactor merge packet number ranges

[q191201771]

Since we iterate ranges from back to front, it is possible to merge two ranges only if extended at the beginning of a range.

1. It's unnecessary to do a merge-check when extended at the end.
2. // if the two ranges were not merge, we're done here, this comment is not matched with the original source code, we're done here because the range was extended(no matter merge or not).

[codecov]

Codecov Report

Merging #3051 (de0a6d4) into master (e3c1a67) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3051      +/-   ##
==========================================
- Coverage   86.19%   86.19%   -0.00%     
==========================================
  Files         132      132              
  Lines        9350     9347       -3     
==========================================
- Hits         8059     8056       -3     
  Misses        935      935              
  Partials      356      356              

Impacted Files	Coverage Δ
internal/ackhandler/received_packet_history.go	96.67% <100.00%> (-0.16%)	⬇️

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[marten-seemann]

Nice!!! I really like this, it makes the code much simpler!
Thank you for the contribution!