Fix PoolBatch index, beginHeight issues and genesis logic

[jaybxyz]

Description

closes: #437

• Remove PoolBatchIndex
• Fix PoolBatch.Index duplicated bug
• Fix PoolBatch.BeginHeight consistency issue on genesis init logic
• Add simulations for genesis export, import
• Update changelog and docs

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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 #438 (5af64ce) into develop (a42fed5) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 5af64ce differs from pull request most recent head ca1087f. Consider uploading reports for the commit ca1087f to get more accurate results

@@             Coverage Diff             @@
##           develop     #438      +/-   ##
===========================================
+ Coverage    85.08%   85.10%   +0.01%     
===========================================
  Files           23       23              
  Lines         2615     2618       +3     
===========================================
+ Hits          2225     2228       +3     
  Misses         226      226              
  Partials       164      164              

Impacted Files	Coverage Δ
x/liquidity/keeper/store.go	90.11% <ø> (-0.44%)	⬇️
x/liquidity/types/keys.go	100.00% <ø> (ø)
x/liquidity/keeper/batch.go	88.81% <100.00%> (ø)
x/liquidity/keeper/liquidity_pool.go	83.65% <100.00%> (ø)
x/liquidity/legacy/v043/helpers.go	100.00% <100.00%> (ø)
x/liquidity/legacy/v043/store.go	100.00% <100.00%> (ø)
x/liquidity/simulation/decoder.go	100.00% <100.00%> (ø)

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