flowcontrol: change variable names for better understanding

[Icarus9913]

Description

This PR aims to improve some variable names for better understanding. Before the change, it took time for users to think about why there's a b variable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.93%. Comparing base (83bead4) to head (10f346c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8578      +/-   ##
==========================================
+ Coverage   80.80%   80.93%   +0.12%     
==========================================
  Files         413      413              
  Lines       40773    40773              
==========================================
+ Hits        32946    32998      +52     
+ Misses       6189     6153      -36     
+ Partials     1638     1622      -16     

Files with missing lines	Coverage Δ
internal/transport/flowcontrol.go	96.49% <100.00%> (ø)

... and 19 files with indirect coverage changes

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