Revise metas to allow multiple keys with same value

[DJPaul]

All components' meta functions should allow multiple keys to be stored with the same value (see WP's add_metadata() $unique).

We should probably also use WP's meta handling functions (add_metadata(), update_metadata(), and so on) to avoid code duplication and ensure meta feature parity.

[DJPaul]

For the second thing, we will probably need to get a filter added to _get_meta_table(). I'll see if WP trac has a ticket. Another approach would be to add the table names to the $wpdb object.

[boonebgorges]

I like the idea of leveraging WP's meta functions, but I think that (as you note) there are some limitations in how table names are built.

I'd like for there to be decent test coverage for the existing functions before attempting to port.

[r-a-y]

Related: #4551

[r-a-y]

Duplicate of #4551.