MODE-1737 Corrected query processing with SELECT DISTINCT and JOIN 

When a SELECT DISTINCT is used within a JOIN query, the processing of the DISTINCT operation results in a failure. This change corrects that operation, allowing the distinct to work correctly with or without a JOIN.
    
Two new test cases were added to first replicate the problem and then verify the correction does fix the problem.

(Note that this pull-request is built on top of [another](https://github.com/ModeShape/modeshape/pull/637) that should be merged first.)