Optimize tablefield_form_alter

tablefield_form_alter() runs for every form on every page. It invokes field_info_instance() for every form and field_info_field() for every table field for every form, even when the form doesn't contain the table field.

Using information saved in $form_state['field'] by the field system it is possible to prevent database calls caused by tablefield_form_alter() on pages that have no table field.