update tendermint/iavl to v0.12.3

[fedekunze]

• 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.

• Wrote tests

• Updated relevant documentation (docs/)

• Added a relevant changelog entry: clog add [section] [stanza] [message]

• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #4718 into master will decrease coverage by 0.01%.
The diff coverage is 11.11%.

@@            Coverage Diff             @@
##           master    #4718      +/-   ##
==========================================
- Coverage   54.15%   54.13%   -0.02%     
==========================================
  Files         272      272              
  Lines       17349    17355       +6     
==========================================
  Hits         9395     9395              
- Misses       7271     7277       +6     
  Partials      683      683