Replace homebrew caching library with 3rd-party caching library

[asmecher]

Suggested: Stash (https://github.com/tedious/Stash).

The object_cache configuration and Memcache support also need review, testing, or removal.

[asmecher]

Another good option (perhaps better): Laravel's caching component https://laravel.com/docs/5.0/cache

[NateWr]

Just a note: you'll probably need to use v5.5, since that's the version we use for the QuerySelector and Validator components: https://laravel.com/docs/5.5/cache

(Usually there's not much difference between 5.* versions.)

[asmecher]

See also https://www.phpfastcache.com/

[asmecher]

(See also the error message noted in #4333 -- replacing the caching library would resolve that. However, it's cosmetic so a low priority.)