Stop AggregateMapper from throwing an error for valid column reference

Previously the AggregateMapper would have issues when a subquery references an outer aggregate but doesn't have an aggregate of it's own. This rearranges some code to first find outer references, then find aggregates within a subquery. 

Conveniently this also fixes a runtime error that occurred under similar circumstances.
It looks like this actually returns rows (5 of them). Did you just want to avoid the unpredictability of NOW()? The `!dc` tag can be used for the expected output in that case.