More robust caching

[stevegrunwell]

As it stands, the caching in Schemify is pretty basic: the object assembled in Thing::build() (for top-level queries only) gets cached indefinitely. If, for instance, an attachment or a user profile were to be updated, these would not be automatically flushed from the cache.

Schemify needs a more robust system, wherein (for instance), an update to an attachment would flush the cache for any post that uses it as a post thumbnail, or a user updating his/her profile would cause the cache for any posts where he/she is the author to be rebuilt.

Naturally, we want to try to make this a non-blocking process that also won't cause a cache stampede on larger sites.