Fix bug in auto_purge_post_cache() that purged the cache unnecessarily

- Previously purged on most statuses, including saving as `draft`. Now only purges post status `publish` and `private`
- Also purges when transitioning from `publish` or `private` post status to `draft`, `future`, or `private`.

See WebSharks/Quick-Cache#43
Cache get_post_status(), use strict post status comparison, and clean things up.
Merge branch 'bug-Quick-Cache-43' into 000000-dev