Support querying for records with multiple terms with AND relation #2768

See #2145: OR and NOT IN handling is present for terms for registered taxonomies, through the use of top-level query parameters for each taxonomy. categories[]=7&categories[]=9 serves as posts with the categories with ID 7 OR 9; AND handling is not yet available. Including tax_query in filter[] was deemed too big a cannon to use to kill this mosquito; this issue tracks revisiting how to handle AND queries for records associated with all of a set of term_ids.