add locking around storage access

[synfinatic]

Refs: #1084

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.43%. Comparing base (6ac9ae9) to head (6f5765a).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1113      +/-   ##
==========================================
+ Coverage   83.21%   83.43%   +0.23%     
==========================================
  Files          35       36       +1     
  Lines        3816     3867      +51     
==========================================
+ Hits         3175     3226      +51     
  Misses        548      548              
  Partials       93       93              

Flag	Coverage Δ
unittests	83.43% <100.00%> (+0.23%)	⬆️

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Files with missing lines	Coverage Δ
internal/storage/flock.go	100.00% <100.00%> (ø)
internal/storage/keyring.go	93.44% <100.00%> (+0.76%)	⬆️

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