Polish code of circuit breaker module

[sczyh30]

Signed-off-by: Eric Zhao [email protected]

Describe what this PR does / why we need it

Polish code of circuit breaker module.

Does this pull request fix one issue?

NONE

Describe how you did it

Improve comments, naming and logging of circuit breaker module.

Describe how to verify it

Run the test cases and demo.

Special notes for reviews

NONE

[codecov-commenter]

Codecov Report

Merging #152 into master will decrease coverage by 0.92%.
The diff coverage is 13.33%.

@@            Coverage Diff             @@
##           master     #152      +/-   ##
==========================================
- Coverage   41.64%   40.71%   -0.93%     
==========================================
  Files          68       68              
  Lines        3314     3313       -1     
==========================================
- Hits         1380     1349      -31     
- Misses       1799     1828      +29     
- Partials      135      136       +1     

Impacted Files	Coverage Δ
core/circuitbreaker/circuit_breaker.go	23.30% <0.00%> (ø)
core/circuitbreaker/stat_slot.go	0.00% <0.00%> (ø)
core/circuitbreaker/rule_manager.go	45.83% <20.00%> (-6.24%)	⬇️
core/circuitbreaker/rule.go	76.19% <33.33%> (-16.20%)	⬇️
core/flow/tc_throttling.go	66.66% <0.00%> (-11.12%)	⬇️

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