Add a test case for handling $set ops with a renamed _id column.
Fix broken test

Ensure the database table is dropped before the test starts.
Look for the proper columns inside of PostgreSQL.
Extend unit tests

Ensure configuration parser supports both the old and the new
syntax.
Add API to retrieve the primary SQL key of a namespace
Fix unit test

A BSON::ObjectId value cannot be stored as integer by PostgreSQL.
Handle properly tables where _id is mapped to a different column

Ensure mosql works fine even when the _id attribute is mapped to a
column with a different name.