Problem: memiavl rootmulti integration is not thread-safe to use

[yihuang]

Closes: #977

Solution:

• add mutex to memiavl.DB.

The assumptions to concurrency:

• The methods on DB are protected by a mutex
• each call of LoadVersion loads a separate instance, in query scenarios, it should be immutable, the cache stores will handle the temporary writes.
• the DB for the state machine will handle writes through the Commit call, this method is the sole entry point for tree modifications, and there's no concurrency internally (the background snapshot rewrite is handled separately), so we don't need locks in the Tree.

👮🏻👮🏻👮🏻 !!!! REFERENCE THE PROBLEM YOUR ARE SOLVING IN THE PR TITLE AND DESCRIBE YOUR SOLUTION HERE !!!! DO NOT FORGET !!!! 👮🏻👮🏻👮🏻

PR Checklist:

• Have you read the CONTRIBUTING.md?
• Does your PR follow the C4 patch requirements?
• Have you rebased your work on top of the latest master?
• Have you checked your code compiles? (make)
• Have you included tests for any non-trivial functionality?
• Have you checked your code passes the unit tests? (make test)
• Have you checked your code formatting is correct? (go fmt)
• Have you checked your basic code style is fine? (golangci-lint run)
• If you added any dependencies, have you checked they do not contain any known vulnerabilities? (go list -json -m all | nancy sleuth)
• If your changes affect the client infrastructure, have you run the integration test?
• If your changes affect public APIs, does your PR follow the C4 evolution of public contracts?
• If your code changes public APIs, have you incremented the crate version numbers and documented your changes in the CHANGELOG.md?
• If you are contributing for the first time, please read the agreement in CONTRIBUTING.md now and add a comment to this pull request stating that your PR is in accordance with the Developer's Certificate of Origin.

Thank you for your code, it's appreciated! :)

[codecov]

Codecov Report

Merging #1002 (276a6bf) into main (ca10265) will increase coverage by 0.22%.
The diff coverage is 70.14%.

@@            Coverage Diff             @@
##             main    #1002      +/-   ##
==========================================
+ Coverage   44.80%   45.02%   +0.22%     
==========================================
  Files          80       80              
  Lines        6606     6669      +63     
==========================================
+ Hits         2960     3003      +43     
- Misses       3322     3342      +20     
  Partials      324      324              

Impacted Files	Coverage Δ
memiavl/db.go	58.46% <70.14%> (+3.13%)	⬆️