fix(app_analytics/cache): use lock to make cache thread safe

[gagantrivedi]

Thanks for submitting a PR! Please check the boxes below:

• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

Use lock to make cache thread safe
fixes #4562

How did you test this code?

Tested manually to make sure two threads can't update _cache update at the same time

[vercel]

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3 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
docs	⬜️ Ignored (Inspect)			Sep 2, 2024 6:24am
flagsmith-frontend-preview	⬜️ Ignored (Inspect)			Sep 2, 2024 6:24am
flagsmith-frontend-staging	⬜️ Ignored (Inspect)			Sep 2, 2024 6:24am

[sentry-io]

🔍 Existing Issues For Review

Your pull request is modifying functions with the following pre-existing issues:

📄 File: api/app_analytics/cache.py

Function	Unhandled Issue
_flush	RuntimeError: dictionary changed size during iteration /api/v1/environment-doc... Event Count: 2

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[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-4567	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-frontend:pr-4567	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-api:pr-4567	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4567	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4567	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-e2e:pr-4567	Finished ✅	Skipped

[github-actions]

Uffizzi Preview deployment-55782 was deleted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.14%. Comparing base (3b20df1) to head (cffc528).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4567   +/-   ##
=======================================
  Coverage   97.14%   97.14%           
=======================================
  Files        1154     1154           
  Lines       39811    39816    +5     
=======================================
+ Hits        38674    38679    +5     
  Misses       1137     1137           

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