Improve caching in get_calendar function

[spacedmonkey]

The get_calendar function is used to generate the calendar. There are a number of things that can be done to improve the performance of this function.

• Replace the raw query to posts table with call to WP_Query query class.
• Change cache invalidation from

  add_action( 'save_post', 'delete_get_calendar_cache' );
  add_action( 'delete_post', 'delete_get_calendar_cache' );
  

to

add_action( 'clean_post_cache', 'delete_get_calendar_cache' );

• Instead of store all caches in a simple cache key, break them up and invalidate caches in another way.

Trac ticket: https://core.trac.wordpress.org/ticket/61343

[narenin]

Hi @spacedmonkey

I have suggested the PR with fix.

@mukeshpanchal27 Could you please take a look.

@spacedmonkey Could you please review this PR.