Invent cache salt

[edneville]

If there are multiple sites using the same redis server then invent a salt that gives uniqueness for each site to avoid collision.

[tillkruss]

This will suddenly give all sites a prefix and jack up their Redis server size. Feel free to extend the documentation, (or admin gui) instead.

[edneville]

@tillkruss, thanks for reviewing. Isn't it better to have a prefix to avoid collision? Would it be acceptable to then to say in the docs to define WP_REDIS_PREFIX with an empty string if that's not desired? Ideally I'd like to provide some default protection to avoid two installs on the same infrastructure from colliding.

[tillkruss]

Yeah, feel free to document it, or even add it to the admin UI.

Remember that the DB prefix isn't unique across multiple installations.