[SPARK-19314][SS][CATALYST] Do not allow sort before aggregation in Structured Streaming plan

## What changes were proposed in this pull request?

Sort in a streaming plan should be allowed only after a aggregation in complete mode. Currently it is incorrectly allowed when present anywhere in the plan. It gives unpredictable potentially incorrect results.

## How was this patch tested?
New test

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #16662 from tdas/SPARK-19314.