delete_post_meta_by_key() fix

[Denis-de-Bernardy]

patch for the todo item is attached

[Denis-de-Bernardy]

The second patch is a suggested enhancement. It replaces the = operator with a LIKE operator, allowing to mass-delete post meta, like:

delete_post_meta_by_key('_plugin_cache_%')

I needed this a couple of time in my own plugins and ended up using SQL to achieve it. Having it in the wp core would be a plus.

[Denis-de-Bernardy]

Second enhanced version lets one use LIKE, NOT LIKE, REGEXP AND NOT REGEXP, as necessary.

[Denis-de-Bernardy]

enhanced and enhanced2 diffs are bad ideas on second though. the first patch is the correct one to use.

[Denis-de-Bernardy]

refreshed against 11255

[ryan]

(In [11305]) Clear cache after deleting meta. Props Denis-de-Bernardy. fixes #9009