Continue to apply QA API implementation feedback

[dongsam]

Description

Apply #173 feedbacks

This PR continues to work after #193

you can see updated swagger v2.2.1

closes: #XXXX

• not clear
  • removed legacy, unofficial base_req, msg.proto #193
  • remove unofficial StdTx for Msg{*}Response
  • remove unofficial StdTx for Msg{*}Response
  • remove unused legacy PoolBatchResponse
• documentation
  • clarify the role of pool_type_id
  • clarify the role of MsgStates

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Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #202 (222a5ae) into develop (63b6a90) will not change coverage.
The diff coverage is n/a.

❗ Current head 222a5ae differs from pull request most recent head a25929d. Consider uploading reports for the commit a25929d to get more accurate results

@@           Coverage Diff            @@
##           develop     #202   +/-   ##
========================================
  Coverage    83.60%   83.60%           
========================================
  Files           20       20           
  Lines         2300     2300           
========================================
  Hits          1923     1923           
  Misses         203      203           
  Partials       174      174           

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