sql: avoid unnecessary singleflight requests for role membership cache

[rafiss]

See individual commits for refactors that make the enhancement easier.

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Since we use a query that scans the whole role_members table to populate the membership cache, it is wasteful to launch multiple singleflight requests for different users. This patch changes the cache refresh logic of the singleflight so that it always populates the entire cache, which means there will only ever be one in-flight query per each node.

fixes https://github.com/cockroachdb/cockroach/issues/135931 Release note (performance improvement): Improved the internal caching logic for role membership information. This reduces the latency impact of commands such as DROP ROLE, CREATE ROLE, and GRANT role TO user, which cause the role membership cache to be invalidated.

[blathers-crl[bot]]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is