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

[mattbucci]

Closes:

• _seq appended to table name, how to deal with it properly? #99
• REPLACE INTO should properly emulate wpsqli_insert_id #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