Improve get_the_post_format_media and get_the_post_format_image caching

[kovshenin]

We need a better approach at caching the results of our get_the_post_format_media and _image functions. Currently we're just dumping the HTML into $post->format_content as a string, so calling:

$post = get_post( 8703 );
echo get_the_post_format_image( 'full', $post );
echo get_the_post_format_media( 'audio', $post );
echo get_the_post_format_media( 'video', $post );

Will output the image tag three times, even if 8703 is an audio, video or quote post. We need to be smarter about how we cache things in structured post formats, and take into account any arguments that are passed in, like the image size for get_the_post_format_image:

$post = get_post( 8703 );
echo get_the_post_format_image( 'thumb', $post );
echo get_the_post_format_image( 'large', $post );
echo get_the_post_format_image( 'full', $post );

Will output three thumbs. Related: #23945.

[kovshenin]

24002.diff​ makes format_content an array and uses a $cache_key to store things inside for get_the_post_format_image and get_the_post_format_media functions.

[lancewillett]

+1

[wonderboymusic]

Refreshed / altered against trunk

[markjaquith]

In 24091:

Smarter structured post format caching.

fixes #24002. props kovshenin, wonderboymusic.