fix dropping of 0-RTT packets

[marten-seemann]

Fixed version of #2749.

Iterate so far didn't allow to remove packets from the packet history, but we still did that for removing 0-RTT packets:
https://github.com/lucas-clemente/quic-go/blob/bed802aee5732c8fe373bf4f71e789bea2f7f11f/internal/ackhandler/sent_packet_handler.go#L158-L166

[codecov]

Codecov Report

Merging #2752 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2752      +/-   ##
==========================================
+ Coverage   86.52%   86.54%   +0.02%     
==========================================
  Files         128      128              
  Lines        9960     9962       +2     
==========================================
+ Hits         8617     8621       +4     
+ Misses       1010     1009       -1     
+ Partials      333      332       -1     

Impacted Files	Coverage Δ
internal/ackhandler/sent_packet_history.go	100.00% <100.00%> (ø)
internal/ackhandler/sent_packet_handler.go	73.54% <0.00%> (+0.43%)	⬆️

---

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 bed802a...ad40c9c. Read the comment docs.