Build rankings service as a web worker

[Lane]

Rankings would perform much better if the service that handles fetching / sorting data was done in a worker. Quite a few changes needed for this, so marking it as low priority and maybe something to do after launch.

[pjsier]

Revisiting this, after seeing the performance with caching on production, do you still think this is necessary? I'm interested in trying out web workers because I've never used them directly, but want to check if the refactoring is worth it. It could make local dev easier even if it doesn't change production performance dramatically. That could also be relevant depending on how the top evictors data is structured

[Lane]

I think it makes sense to do this as a refactoring task when we build top evictors. Not sure if it's absolutely necessary right now though, performance seems alright on production to me.