Rule Manager: Improve CircuitBreaker rule loading API (#236)

[TimDu]

Describe what this PR does / why we need it

PR per issue #236

Does this pull request fix one issue?

Add fixes to solve issue #236

Describe how you did it

Add logic to check for existing rule changes in this API and return as the first value
Put recover method in one place so that all panics can be caught and assigned to the second value
Failed rules will be returned to caller as the third value
Add duplicate rule checker so that all loaded rules will be unique

Describe how to verify it

Added more UT cases to verify it

Special notes for reviews

Also Add CB Strategy Enumeration upper boundary.
Modify API signature

[CLAassistant]

All committers have signed the CLA.

[louyuting]

Could you please fix CI?

[codecov-commenter]

Codecov Report

Merging #257 into master will increase coverage by 0.66%.
The diff coverage is 86.95%.

@@            Coverage Diff             @@
##           master     #257      +/-   ##
==========================================
+ Coverage   43.06%   43.73%   +0.66%     
==========================================
  Files          79       79              
  Lines        3994     4020      +26     
==========================================
+ Hits         1720     1758      +38     
+ Misses       2009     2003       -6     
+ Partials      265      259       -6     

Impacted Files	Coverage Δ
core/circuitbreaker/rule_manager.go	54.00% <86.04%> (+12.62%)	⬆️
core/circuitbreaker/rule.go	36.66% <100.00%> (+6.66%)	⬆️
ext/datasource/helper.go	55.78% <100.00%> (ø)

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[TimDu]

Thanks @louyuting for your update. I did only send out changes on CircuitBreaker module since there are multiple changes to Flow module in the latest branch. Would love to discuss and help on other modules. Please let me know if any concerns on current implementation.

[louyuting]

Thanks @louyuting for your update. I did only send out changes on CircuitBreaker module since there are multiple changes to Flow module in the latest branch. Would love to discuss and help on other modules. Please let me know if any concerns on current implementation.

That's great. I will review this PR. Because of holiday, the response might be later.