Allow ordering by meta_id in a meta query

[jamieburchell]

It is currently not possible to order by wp_postmeta.meta_id during a meta query. This would be useful because it allows rows to return in the order they were inserted into the table.

A real use case example is where a custom field plugin exposes a multiple "Add item" mechanism with the option to reorder them. Pods for example does this with their relationship type field with multiple selection and reordering. The meta data is then inserted in to the postmeta table in the desired order.