Fix potential race conditions with InMemoryStore accesses

[colinodell]

This PR introduces a sync.RWMutex to the InMemoryStore, ensuring safe concurrent access to the in-memory slice of users. This change addresses potential race conditions that can arise when multiple goroutines read and modify the slice at the same time.

[bbckr]

Surprised we din't already have that, great change!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.75%. Comparing base (9b58328) to head (b78755e).
Report is 1 commits behind head on main.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
+ Coverage   77.54%   77.75%   +0.20%     
==========================================
  Files          20       20              
  Lines         971      980       +9     
==========================================
+ Hits          753      762       +9     
  Misses        205      205              
  Partials       13       13              

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