Improves the performance of the Redis set cache

What does this MR do?

Part of gitlab-com/gl-infra/scalability#48 (closed)

Improves the performance of the Redis set cache

Testing has shown that treating membership optimistically reduces the number of Redis calls substantially.

• include? will only perform an EXISTS if SISMEMBER returns negative.

• fetch will only perform an EXISTS if there are no values.

• read uses SSCAN to avoid Redis lockup on huge sets

include? Before

Two Redis calls, 10 milliseconds roundtrip per item

include? After

Usually one Redis call, 5 milliseconds roundtrip per item

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team