[Building Sync-ups Tutorial] fixes in "Persisting sync-ups" chapter

[FredericRuaudel]

Hi!

Following my last PR, I'm currently reviewing the "Building Sync-ups" tutorial and so far, there is a lot of changes so I've splitted up into one PR per chapter to ease the review of them, continuing with the "Persisting sync-ups" chapter here…

Changes Made:

• "Persisting data to disk" Tutorial Section Enhancements:

  • Implemented thread-safe mutations of the shared syncUps using withLock.
  • Updated the tutorial code to compile correctly with the new file storage approach.

• "Testing persistence" Tutorial Section Improvements:

  • Moved the initialization of syncUps outside of the TestStore to prevent shared state issues during testing.
  • Incorporated withLock for thread-safe mutations of shared state.
  • Enhanced the tutorial explanation to provide clearer guidance on the fixes implemented.

Doing so, I needed to add new steps in the tutorial description and so new code files too. I tried my best to match your tone and vocabulary in the description of the new steps but feel free to enhance the copyrighting if you need to.