Rework cache.Cache to use x/sync/singleflight

[rowanseymour]

No description provided.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (877c587) 90.05% compared to head (250193d) 90.03%.

Files	Patch %	Lines
cache/generic.go	88.23%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #113      +/-   ##
==========================================
- Coverage   90.05%   90.03%   -0.03%     
==========================================
  Files          40       40              
  Lines        1940     1936       -4     
==========================================
- Hits         1747     1743       -4     
- Misses        153      155       +2     
+ Partials       40       38       -2     

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[rowanseymour]

I can only get a thread here if I add a small random delay in Get between the cache lookup and the call to fetch and set, e.g. time.Sleep(time.Duration(random.IntN(100)) * time.Millisecond)