Introduce a caching layer in `bu_navigation_get_posts`

[philcable]

https://trello.com/c/eU9qfTAa/30-ca-6-bu-navigation

This approach leverages keys stored in a new cache group - bu-navigation-persistent - as bu-navigation is set as non-persistent.

A copy of the last_changed cache key value from WP core is stored along with the results of the query made in the bu_navigation_get_posts function. This is stored with a key that uses the arguments passed to the function, JSON encoded and wrapped in md5, so that multiple unique calls to the function will retrieve their respective results.

If the last_changed value stored with the results of the query don't match the current WP core value of last_changed, the cache for that call is reset.

WP core's last_changed key is also reset whenever changes to the menu are saved.

I think this approach makes sense, but I appreciate that my understanding of the plugin as a whole may not be complete enough, and I could be missing something.