## What changes were proposed in this pull request?
All streaming logical plans will now have isStreaming set. This involved adding isStreaming as a case class arg in a few cases, since a node might be logically streaming depending on where it came from.

## How was this patch tested?

Existing unit tests - no functional change is intended in this PR.

Author: Jose Torres <joseph-torres@databricks.com>
Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #18973 from joseph-torres/SPARK-21765.