Eng 6597 union parameters

I was thinking about this over the weekend.  In this case I think I would prefer a protected accessor in the base class (setParamList), and have the comment appear before a call to setParamList in ParsedUnionStmt's parse method.  In general, if I want to learn about how unions are handled, I'll be looking at ParsedUnionStmt.java.  

I guess I'm okay with a generic accessor opening up access to the param list to the subclasses.  It seems like a reasonable thing for subclasses to manipulate, and if I ever want to know "hey, who changed this param list?", then the accessor provides a nice place to set a breakpoint.

Another way to handle this might be to refactor the code in AbstractParsedStmt's method parseTablesAndParams to allow some virtual method to be called in ParsedUnionStmt for parameter handling.  But I'm not sure if that's worth the trouble---specifically I don't know if the subqueries being unioned have been parsed yet, so their parameter lists may not yet be available.

Just my thoughts.  I'm fine with these changes being merged to master.