[Bugfix] fix to use single wasmvm during recursive external query

[yun-yeo]

Summary of changes

close #587

Also previously, only single wasmvm was used because wasm keeper did not maintain reference of ReadVMPool.

Report of required housekeeping

• Github issue OR spec proposal link
• Wrote tests
• Updated API documentation (client/lcd/swagger-ui/swagger.yaml)
• Added a relevant changelog entry: clog add [section] [stanza] [message]

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(FOR ADMIN) Before merging

• Added appropriate labels to PR
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)
• Confirm added tests are consistent with the intended behavior of changes
• Ensure all tests pass

[codecov]

Codecov Report

Merging #588 (dc8e571) into main (8804d48) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #588      +/-   ##
==========================================
+ Coverage   44.75%   44.78%   +0.02%     
==========================================
  Files         122      122              
  Lines        7013     7016       +3     
==========================================
+ Hits         3139     3142       +3     
  Misses       3628     3628              
  Partials      246      246              

Impacted Files	Coverage Δ
x/wasm/types/keys.go	0.00% <ø> (ø)
x/wasm/keeper/contract.go	79.00% <100.00%> (+0.16%)	⬆️
x/wasm/keeper/keeper.go	81.81% <100.00%> (ø)
x/wasm/keeper/legacy_querier.go	35.22% <100.00%> (+0.74%)	⬆️
x/wasm/keeper/pool.go	83.33% <100.00%> (ø)
x/wasm/keeper/querier.go	35.38% <100.00%> (ø)