Replace calls to seq by adding a RETURNING * to all insert and replace statements

Closes:

https://github.com/PostgreSQL-For-Wordpress/postgresql-for-wordpress/issues/99
https://github.com/PostgreSQL-For-Wordpress/postgresql-for-wordpress/issues/108

This PR appends RETURNING * to all insert queries so that wpsqli_insert_id can check for a primary key on the table that was last inserted against and grab the last inserted value in a more robust way, especially for tables without sequences.

Removes of noop logic for term_relationships
Removes custom logic for handling post_author insertion

PostgreSQL-For-Wordpress / postgresql-for-wordpress

Replace calls to seq by adding a RETURNING * to all insert and replace statements #101