[SPARK-21988] Add default stats to StreamingExecutionRelation.

[jose-torres]

What changes were proposed in this pull request?

Add default stats to StreamingExecutionRelation.

How was this patch tested?

existing unit tests and an explain() test to be sure

[SparkQA]

Test build #81695 has finished for PR 19212 at commit 80d0268.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• case class StreamingExecutionRelation(

[jose-torres]

@zsxwing for review

[zsxwing]

LGTM pending tests

[SparkQA]

Test build #81738 has finished for PR 19212 at commit 81b1af9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[zsxwing]

Thanks! Merging to master.