Fix nack buffer RTX issue

[kcaffrey]

There was previously an issue with the nack buffer that would occur when an uploader was experiencing packet loss (or otherwise had reordering occur). lastAdded was getting set for every packet added to the buffer, but packets could get added out of order, causing lastAdded to go backwards. The impact was that packets at the end of the buffer were sometimes considered stale and cleared out. Consider the following sequence of packets being added to the buffer:

1, 2, 4, 3, 5, ...

When 4 is added, lastAdded would correctly be 4, but then when 3 arrived, lastAdded would go backwards to 3. This would cause two related problems: trying to get 4 from the buffer would fail (uint16(3 - 4) > Uint16SizeHalf), and then when we add 5, we would treat 4 as a gap between lastAdded (3) and seq (5) and thus release 4 from the buffer, such that even after 5 was added, we would still not be able to retrieve 4 from the buffer.

This PR modifies the logic to only move lastAdded forward if diff is "positive" (less than Uint16SizeHalf), meaning the new packet is newer than any packet received previously.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.20%. Comparing base (9251977) to head (0c40de4).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
+ Coverage   71.14%   71.20%   +0.06%     
==========================================
  Files          80       80              
  Lines        4477     4477              
==========================================
+ Hits         3185     3188       +3     
+ Misses       1157     1155       -2     
+ Partials      135      134       -1     

Flag	Coverage Δ
go	71.00% <100.00%> (-0.14%)	⬇️
wasm	69.35% <100.00%> (+0.13%)	⬆️

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[boks1971]

lgtm!