Add one MTU headroom to pacing burst

[mengelbart]

This adds one MTU of headroom to the pacing burst. Without it, the burst will be clamped every interval. That is a problem when the pacer has left over tokens but not enough for a complete packet. The leftover tokens should be usable in the next interval, but they're clamped by the max burst, so we drop them instead of using them on the next packet.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.52%. Comparing base (13100ce) to head (0f12f31).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #457      +/-   ##
==========================================
+ Coverage   81.44%   81.52%   +0.08%     
==========================================
  Files          88       88              
  Lines        4656     4656              
==========================================
+ Hits         3792     3796       +4     
+ Misses        686      684       -2     
+ Partials      178      176       -2     

Flag	Coverage Δ
go	81.52% <100.00%> (+0.08%)	⬆️

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