1st Draft Patch for DERBY-754 (pushing Join predicates down to subqueries)

[Army]

I have attached the first DRAFT of a patch for DERBY-754 to that Jira issue.  I 
still have testing to do and I need to add appropriate tests to the derbyall 
suite, so this patch is *NOT* for commit.  However, as the issue of pushing join 
predicates is rather complicated, I'm hoping people out there can offer an early 
review of what I've done so far, so that I can see if I'm at least headed in the 
right direction.

I'll be posting a final patch after I receive feedback on this draft, and after 
I run more tests (and add those tests to derbyall).  In the meantime, I would 
appreciate any comments/corrections.

Please see DERBY-754 for more details.

Thanks,
Army