AO3-6706 Support Sentinel config for Redis clients

[brianjaustin]

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Issue

https://otwarchive.atlassian.net/browse/AO3-6706

Purpose

With some work I've done (and will do after this is merged) on the systems side, this will let us use Redis Sentinel to support automatic failovers when one Redis server becomes unavailable. This is better for High Availability and situations where we need to upgrade software/hardware.

For reviewers: I'd recommend turning whitespace diff off, since I made changes there to satisfy Rubocop.

Credit

Brian Austin (they/he)

[Bilka2]

Code looks good.

Sidenote, the example config in the code comment is invalid:

• The docs show that name should be set in the redis options when using sentinels.
• The port of the second sentinel is too high for a port number.