Reduce write DB querys on post update - change from delete and add new - to check for changes

[quasel]

Current Behavior:
delete_post_meta( $post_id, $field_name ) and then does an
add_post_meta( $post_id, $field_name, $value_id )

it needs to get the meta first (no conflict mode on, so we don't interfere with our hooks), find out what IDs are there, then insert the ones not already there

as discussed with  @sc0ttkclark in slack