Eng 5910 insert into select

These changes implement single-partition stored procedure support for insert into select.

Yes, I tried this and it worked without any trouble.  In general StmtSubquery's method looks more well-baked, so I will remove this method from PlanAssembler.  

I wonder if there is a better place for this method?  It seems to be of more general use now that it's used by both PlanAssembler and StmtSubquery.