Problem: tx is not replaced by priority

[yihuang]

Solution:

• Add TxReplacement callback

Closes: #XXX

Description

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For contributor use:

• 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

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 61.55%. Comparing base (fe3f4fd) to head (e195207).

Files with missing lines	Patch %	Lines
app/app.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #519      +/-   ##
===========================================
- Coverage    61.57%   61.55%   -0.03%     
===========================================
  Files          128      128              
  Lines         9512     9514       +2     
===========================================
- Hits          5857     5856       -1     
- Misses        3115     3117       +2     
- Partials       540      541       +1     

Files with missing lines	Coverage Δ
app/app.go	75.92% <0.00%> (-0.45%)	⬇️