[rfc] move caches into redis

[hawkrives]

thoughts?

seems like that's kinda … one of redis' specialities.

downsides: added dependency, but Docker/-compose should make that almost painless…

[rye]

I am wholeheartedly in favor of this.

Just expanding on a couple of points.

Pros

• Caches that persist continuously through ccc, et al. updates.
• Caches that could be shared between all of our containers.
• Cache expiration handled natively by redis internally.
• Redis can handle storing Strings, Hashes, Lists, Sets, Sorted Sets.
• Depending on client support for it, Redis also supports pipelining.

Cons

• Potential for added latency (rather than SHM, we'd be shelling out to an "external" service)
• Added dependency (both in each project that consumes it, and also on the server)