reduce calls to time.Now() from the flow controller

[marten-seemann]

No description provided.

[codecov]

Codecov Report

Merging #2591 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2591   +/-   ##
=======================================
  Coverage   86.27%   86.28%           
=======================================
  Files         122      122           
  Lines        9755     9756    +1     
=======================================
+ Hits         8416     8417    +1     
  Misses        997      997           
  Partials      342      342           

Impacted Files	Coverage Δ
internal/flowcontrol/base_flow_controller.go	100.00% <100.00%> (ø)
internal/flowcontrol/connection_flow_controller.go	100.00% <100.00%> (ø)

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