Cache posthog events on S3 or some other persistent storage instead of caching them on disk

[Martinsos]

Right now we cache fetched posthog events on disk, in a .json file.
Problem is, in production, when deployed to Heroku, this file is lost whenever a dyno (Heroku instance) is restarted, which can happen quite often on Heroku, possibly even daily (Heroku restarts dynos often to keep them "healthy").

Solution is to store the file somewhere on persistent storage -> most likely on S3.