Redis: Unix domains sockets/per-user redis instances

[fierevere]

It will be nice to change Redis chapter - https://make.wordpress.org/hosting/handbook/handbook/security/ and include information that users may have their own per-user instances of redis-server running, on different ports or Unix socket (which is most secure approach without credentials authentification). However Redis object caching plugins will need to have a way to use changed port or connect via Unix socket, instead of TCP