Feature request: make nodes recognize the 'time' value

Hi,

I couldn't not find how to make nodes like `changeDetect(field string)` or functions like `distinct(field string)` take the time into account.

Can we support the special 'time' value who would pick it out from the time?

I understand this would be an issue if someone sends data with 'time' in the fields.

My use-case is deduplicating data when using influx-relay and influxdb subscriptions: every influxdb instance sends data to the same kapacitor, which receives data multiple times.

Counts like stateCount get messed up by the duplicate data.

Best,
Anthony

Begin at the changeDetect(field string) and distinct(field string) entry points named in the issue, then trace how fields are selected. Done means the special 'time' value is recognized for these operations and duplicate InfluxDB points in the stated use case are handled without breaking ordinary fields.

Written by the indexing model from the issue text.