serious overhaul to get eager loads to work inline with an inheriting mapper, when...

serious overhaul to get eager loads to work inline with an inheriting mapper, when the inheritance/eager loads share the same table.  mapper inheritance will also favor the columns from the child table over those of the parent table when assigning column values to object attributes.  "correlated subqueries" require a flag "correlated=True" if they are in the FROM clause of another SELECT statement, and they want to be correlated.  this flag is set by default when using an "exists" clause.