Implement `WP_CACHE_KEY_SALT` const

[spacedmonkey]

The WP_CACHE_KEY_SALT const is used to cache invalidate. Core uses and many popular plugins use it as well. This plugin should use it as well.

[OllieJones]

Each site (or network containing multiple sites) creates its own cache .sqlite file. The primary purpose of this "salt" is to tag entries from different sites/networks that share a single cache. Agreed that it should be there to meet user expectations, but it is, strictly speaking, not necessary.

If a future version of this allows the sharing of .sqlite files by multiple sites/networks, this will become useful.

[spacedmonkey]

There are a couple of ways to implement this.

1. Add the salt to the key generation. See
2. Prefix the file name with salt.

WP_CACHE_KEY_SALT is used by all of the popular object caching drop-ins. Even wp-redis, which this plugin is based on.

[OllieJones]

We append the salt to the cache's SQLite filename. (There's nothing to be gained by lengthening the cache item names, because each .sqlite file is used for only one install anyway.