Allow differing aggregate functions to apply to different columns of the same table.

[adamperlin]

From ifql created by nathanielc : influxdata/ifql#207

Currently only a single aggregate function can be applied to a window. If multiple aggregates are desired one must perform each aggregate separately and then join the results back together.

In other words the user should not need to rejoin a table that has already been joined.

In addition the _time column should be available for aggregation. https://github.com/influxdata/influxdb/issues/5369

[nathanielc]

This will be possible via this helper function https://github.com/influxdata/platform/issues/139

[yenicelik]

Was this resolved by now? The above issue seems to lead to a dead link