With javitonino's help, greatly reduce the processing time by using
EXCEPT instead of NOT IN, which causes it to use a `HashSetOp Except`
plan on the subqueries rather than a `Seq Scan` on `Materialize`'d
subtables.
It assumes there's a cartodb_id column in both source and target. It
does not perform unnecessary actions. It respects augmented columns in
target table, if they exist. It is meant to be efficient.
