[Proposal] Improve singleflight pacakge

[hlts2]

Is your feature request related to a problem? Please describe.:

The current implementation uses sync.Mutex and map to cache objects.
Removing the lock logic will improve performance.

Describe the solution you'd like:

Since locks and unlocks occur frequently, replacing them with sync.Map can be made lock-free, which is considered to speed up the process.

Describe alternatives you've considered:

Change from sync.Mutex and map to sync.Map to cache object.

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[vankichi]

colse