[staking] clean up internal interface

[dustinxie]

Description

as title

Type of change

Please delete options that are not relevant.

• Code refactor or improvement

How Has This Been Tested?

• make test

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[dustinxie]

these func should go to candSR and candSM, and don't need to be exported

[codecov]

Codecov Report

Merging #3404 (323d09a) into master (180ab52) will not change coverage.
The diff coverage is 90.90%.

@@           Coverage Diff           @@
##           master    #3404   +/-   ##
=======================================
  Coverage   75.32%   75.32%           
=======================================
  Files         236      236           
  Lines       21973    21973           
=======================================
  Hits        16551    16551           
  Misses       4530     4530           
  Partials      892      892           

Impacted Files	Coverage Δ
action/protocol/staking/bucket_index.go	93.33% <ø> (-1.91%)	⬇️
action/protocol/staking/candidate_statereader.go	57.31% <66.66%> (+0.68%)	⬆️
action/protocol/staking/candidate_statemanager.go	97.22% <100.00%> (+0.16%)	⬆️
action/protocol/staking/handlers.go	74.83% <100.00%> (ø)

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