Clear site cache when WP Core is upgraded, or an active theme or plugin

Resolves websharks/quick-cache#327
Enhancing `upgrader_process_complete()`. See: https://github.com/websharks/quick-cache/pull/330
Use $upgrader_instance to check plugin state with a single-plugin update
Build a reference to \Plugin_Upgrader_Skin for IDEs
Merge branch 'feature/327' into 000000-dev