s3: fix race conditions

[neolynx]

Fixes: #1223

Description of the Change

Path Cache handling was already addressed in PR #1224 (which was not merged), #1241 (merged instead).

Make sure pathCache is properly locked for concurrent access.

Add RWMutex to the PublishedStorage struct and locked every access to pathCache.

• Cache initialization
  Read-lock to test for nil, then write-lock with a second nil check before populating
• Cache reads
  RLock/RUnlock, allowing concurrent readers
• Cache writes / deletes
  Lock/Unlock

cc: @sf-nwaller

[codecov]

Codecov Report

❌ Patch coverage is 82.14286% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.37%. Comparing base (c0c4bc3) to head (c633247).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
s3/public.go	80.76%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1594   +/-   ##
=======================================
  Coverage   77.37%   77.37%           
=======================================
  Files         165      165           
  Lines       15727    15747   +20     
=======================================
+ Hits        12168    12185   +17     
- Misses       2353     2356    +3     
  Partials     1206     1206           

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