Correct usage of Mul/Quo and Mul/QuoTruncate

[hallazzang]

Description

• use Mul/Quo for pool price(reserve ratio)
• use *Truncate for minting pool coin amount

closes: #380

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• 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 #413 (5948d0b) into develop (2a38c43) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #413   +/-   ##
========================================
  Coverage    83.45%   83.45%           
========================================
  Files           20       20           
  Lines         2278     2278           
========================================
  Hits          1901     1901           
  Misses         215      215           
  Partials       162      162           

Impacted Files	Coverage Δ
x/liquidity/keeper/liquidity_pool.go	82.08% <100.00%> (ø)

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