Parquet tables don't respect primary keys

[rebasedming]

Bug Description
It is possible to insert duplicate values into a primary key column.

How To Reproduce

CREATE TABLE users (id integer primary key, name text, email text) USING parquet;
INSERT INTO users(id, name, email)
VALUES(1, 'John Doe', 'john@example.com');
-- This should not be allowed, but it is
INSERT INTO users(id, name, email)
VALUES(1, 'John Doe', 'john@example.com');
-- Now we have a duplicate primary key
SELECT * FROM users;
 id |   name   |      email
----+----------+------------------
  1 | John Doe | john@example.com
  1 | John Doe | john@example.com
(2 rows)

Proposed Fix