Support adding persistence to the existing redis instance

We should persist at least the horizon data as well, and we currently
run with "lossy persistence", just not on pvc. With the data that we do
store in redis, we can generally afford to loose a bit of data in the unusual case
where redis restarts, so snapshots are good enough (for laravel caching,
horizon, roundcube caching and spamassassin).
We could in a second iteration add a pure in-memory cache as a
performance optimization, but this is probably good enough.