optimises a call to fetch all post meta in the duplication code

[tomjn]

Changes proposed in this Pull Request:

• This swaps out an SQL query for get_post_meta with a single argument

This should simplify the code, and improve performance via WP_Cache. The biggest improvements are likely to be seen when a persistent object cache is available

Testing instructions:

• duplicate listings

Proposed changelog entry for your changes:

• Optimisations to listing duplication

[jom]

As is, this breaks duplication functionality. I think this is because get_post_meta() returns values in an array and we treat these as unique (so just want the first one, if it exists).

In this example, post_id = 1277 is the duplicate and has its value set as a serialized array.

My other concern is about plugin interactions with filters in get_metadata(), such as get_{$meta_type}_metadata. I'm not sure if WPML or other plugins that do things with metadata in view would prevent us from getting the raw value.