feat: call Close method on app to cleanup resource on graceful shutdown

[yihuang]

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

Merging #246 (f74ccad) into release/v0.20.x-cronos (72bbe0a) will increase coverage by 4.12%.
The diff coverage is n/a.

❗ Current head f74ccad differs from pull request most recent head 4ba39b2. Consider uploading reports for the commit 4ba39b2 to get more accurate results

@@                    Coverage Diff                     @@
##           release/v0.20.x-cronos     #246      +/-   ##
==========================================================
+ Coverage                   52.56%   56.68%   +4.12%     
==========================================================
  Files                         113      108       -5     
  Lines                       11073    10243     -830     
==========================================================
- Hits                         5820     5806      -14     
+ Misses                       5006     4190     -816     
  Partials                      247      247              

see 5 files with indirect coverage changes