[SPARK-20441][SPARK-20432][SS] Within the same streaming query, one...

[SPARK-20441][SPARK-20432][SS] Within the same streaming query, one StreamingRelation should only be transformed to one StreamingExecutionRelation

## What changes were proposed in this pull request?

Within the same streaming query, when one `StreamingRelation` is referred multiple times – e.g. `df.union(df)` – we should transform it only to one `StreamingExecutionRelation`, instead of two or more different `StreamingExecutionRelation`s (each of which would have a separate set of source, source logs, ...).

## How was this patch tested?

Added two test cases, each of which would fail without this patch.

Author: Liwei Lin <lwlin7@gmail.com>

Closes #17735 from lw-lin/SPARK-20441.