fix: cachedFetcher.update() no longer blocks token reads during DB refresh

[NETIZEN-11]

What was fixed

The cachedFetcher.update() function in token/services/selector/sherdlock/fetcher.go was holding a write lock for the entire duration of the database query. This meant whenever the cache needed to refresh (e.g., on a slow DB), all token read operations would block waiting for the lock.

How it was fixed

• Released the lock before executing the DB query
• Added a staleness re-check after the DB call completes, to avoid overwriting a cache that another goroutine may have already refreshed
• The cache update still happens atomically once the lock is re-acquired

Tests added

• TestCachedFetcher_UpdateDoesNotBlockReaders: verifies that concurrent readers can still acquire the lock while update() is waiting on the database
• TestCachedFetcher_UpdateReacquiresLockAfterDB: verifies the cache is correctly updated after the lock is re-acquired

go test -v -run "TestCachedFetcher_UpdateDoesNotBlockReaders|TestCachedFetcher_UpdateReacquiresLockAfterDB" ./token/services/selector/sherdlock/...

[NETIZEN-11]

@adecaro Would love a review on this. Thanks!

[adecaro]

Hi @NETIZEN-11 , thanks for submitting this 🙏
I'll review ASAP but, please, open a Github Issue related to this.

Thanks much 🙏

[adecaro]

@NETIZEN-11 , please, run make lint-auto-fix. It should solve the current lint errors 🙏