redis: Tune settings for `cache` and `store`

Created by: tsenart

This commit reverts the part of #8114 that made redis-cache not persistent. Sourcegraph cannot work well with a cold cache, so we need to persist this cached data to prevent cold starts (e.g. for GitHub and GitLab cached user permissions).

It also tunes Redis settings specifically for cache and store to match what we have in the kubernetes deployments:

• store: We change the maxmemory-policy setting to noeviction.
• cache: Since it's ephemeral, we don't use AOF persistence, but reduce the RDB snapshot interval to 60 seconds — this means we have a data loss window of 1 minute, which should be fine for cache, specially since persistence only optimizes the cold cache scenario.