[1.x,2.x] Subqueries do not correctly handle aggregation functions

It seems Influx (v1.8) doesn't handle aggregation functions on subqueries correctly, or I'm misunderstanding how they should behave.

__Steps to reproduce:__
List the minimal actions needed to reproduce the behaviour.

1. Make a query with a LIMIT, return a handful of rows.
2. Make a query with an aggregator function, using the first query in the FROM.

__Expected behaviour:__
The aggregator function is correctly applied to the rows returned by the subquery.

__Actual behaviour:__
I have no idea what calculation is done now, but it's not the expected result.

__Environment info:__

* Version: 1.8

Example:

I would expect the sum of 1 + 1 + 0 + 0 + 0 to be 2, not 5.