kaiax/gov: Rename getters

[ian0371]

Proposed changes

Rename getters

• EffectiveParamSet -> GetParamSet
• EffectiveParamsPartial -> GetPartialParamSet

Others

• bugfix: lowestVoteScannedBlockNumKey sometimes stored block num or epoch idx. It's been renamed to lowestVoteScannedEpochIdxKey and now stores epoch idx only.
• schema.go won't use lock for a single DB lookup.
• Unit tests has been added.
• Migration goroutine stops gracefully.
• The redundant first parameter from AddVote() has been removed.

Types of changes

• Bugfix
• New feature or enhancement
• Others

Checklist

• I have read the CONTRIBUTING GUIDELINES doc
• I have read the CLA and signed by comment I have read the CLA Document and I hereby sign the CLA in first time contribute
• 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

Related issues

Follow up of #177.