nixos/redis: unbreak module

The redis module currently fails to start up, most likely due to running
a chown as non-root in preStart.

While at it, I hardcoded it to use systemd's StateDirectory and
DynamicUser to manage directory permissions, removed the unused
appendOnlyFilename option, and the pidFile option.

We properly tell redis now it's daemonized, and it'll use notify support
to signal readiness.