Can't use autoincrementIdField with PostgreSQL and vocabulary schema

Whenever you declare an SQL directory with an 'id' field and autoincrementIdField is set to true, the related schema element is set to an integer type. This is logicial since I guess most or all backends can only automatically increment a field if it's an integer.

This breaks the hardcoded assumption that the 'id' field in a vocabulary schema is an string and prevents using such a vocabulary on the UI layer of Nuxeo.

I understand that this may be a duplicate of NXP-4935 but since I could not find any report of such a problem in the bug tracker, I decided to write this report in the interest of people facing the same limitation.