Allow sites to skip a query in meta_form() that can delay loading the page

[kitchin]

Follow-up to #24998 (and its duplicates #32361, #32449). See also #18979.

Edit post loads slowly on sites with large tables. Sites should be able to bypass the meta drop-down query. I'm attaching a simple patch using the existing filter postmeta_form_limit.

Some sites do not need the drop-down, or have too many custom metas to make a drop-down useful. The drop-down is great for me though. I can easily add features without creating a custom meta box. Once an option is in the drop-down, my authors know it is available.

[kitchin]

By the way, a workaround is foreach ( get_post_types() as $type ) remove_post_type_support( $type, 'custom-fields' );. But my patch is sensible and keeps the meta box.

[kitchin]

Correction: Follow-up to #24498 not #24998

[peterwilsoncc]

This was discussed in a triage session today.

Avoiding connecting to the database for a LIMIT 0 query makes sense.

For sites that do set the limit to display zero keys using the postmeta_form_limit filter, the ​MySQL optimizer will skip the reading the table so the sub-optimal where clause won't slow things down.

33296.patch​ no longer applies cleanly so adding the needs-refresh keywords.