Factor out redis store duplication

What does this MR do?

In preparation for gitlab-com/gl-infra/scalability#1058 (closed), this MR improves test coverage and removes code duplication between the current three Redis stores we have. This will make it easier to add a fourth one.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Security

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