privatize some methods in the congestion controller package

[marten-seemann]

No description provided.

[codecov]

Codecov Report

Merging #2681 into master will increase coverage by 0.01%.
The diff coverage is 88.89%.

@@            Coverage Diff             @@
##           master    #2681      +/-   ##
==========================================
+ Coverage   86.74%   86.75%   +0.01%     
==========================================
  Files         124      124              
  Lines        9897     9889       -8     
==========================================
- Hits         8585     8579       -6     
+ Misses        978      976       -2     
  Partials      334      334              

Impacted Files	Coverage Δ
internal/congestion/cubic_sender.go	92.13% <88.89%> (+1.01%)	⬆️

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