Rename flow.FlowRule to flow.Rule and remove unused cluster fields

[louyuting]

Describe what this PR does / why we need it

1. remove cluster flow control code related.
2. rename flow.FlowRule to flow.Rule

Does this pull request fix one issue?

Describe how you did it

Describe how to verify it

Special notes for reviews

[codecov-commenter]

Codecov Report

Merging #221 into master will decrease coverage by 0.06%.
The diff coverage is 52.00%.

@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
- Coverage   42.63%   42.56%   -0.07%     
==========================================
  Files          84       84              
  Lines        4492     4485       -7     
==========================================
- Hits         1915     1909       -6     
+ Misses       2334     2333       -1     
  Partials      243      243              

Impacted Files	Coverage Δ
core/flow/rule.go	0.00% <0.00%> (ø)
core/flow/slot.go	0.00% <0.00%> (ø)
core/flow/tc_default.go	0.00% <0.00%> (ø)
core/flow/tc_warm_up.go	0.00% <0.00%> (ø)
core/flow/rule_manager.go	51.58% <58.33%> (-2.21%)	⬇️
ext/datasource/helper.go	57.65% <80.00%> (ø)
core/flow/traffic_shaping.go	36.36% <100.00%> (ø)

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[sczyh30]

LGTM

[sczyh30]

Thanks!