Stream data for aggregations without grouping

[fpetkovski]

When running an aggregation like sum(metric), we can stream all series into an aggregate instead of blocking to fetch data before we start execution. The aggregate can compute values for evaluation steps one series at a time instead of one step at a time.

The reason why we can do this is because the end result will be guaranteed to have a cardinality of 1 so holding intermediary per-step aggregates in memory will be fairly cheap.