Allow user to override set distinct on jdo queries w/ implicit joins

[djerraballi]

Feature Request: provide as much detail as possible about behaviour

Many SQL datastores take a performance hit when running a query w/ a distinct clause. If a user's application can determine that their queries guarantee distinct results without the distinct clause, the application could receive a performance benefit by having that clause removed.

#123

[andyjefferson]

Pull request applied. Thx