Code and comments cleaning for Randao and BLS

[aidan-kwon]

Proposed changes

• Remove headerMixHash and add validation logic in CalcRandao and VerifyRandao
  • headerMixHash could return nil value and it can cause panic in VerfiyRandao
• Rename getAllCached to getBlsInfos
• Remove unnecessary if conditions in snapshot.apply
• Copy and assign valset.mixHash only when it is not empty
  • Since weightedCouncil.Copy() can make an empty slice for mixHash, and it make hard to checking nil value for mixHash
• Add more comments

Types of changes

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• Bugfix
• New feature or enhancement
• Others

Checklist

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• I have read the CONTRIBUTING GUIDELINES doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes ($ make test)
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules