[SPARK-22861][SQL] SQLAppStatusListener handles multi-job executions.

[squito]

When one execution has multiple jobs, we need to append to the set of
stages, not replace them on every job.

Added unit test and ran existing tests on jenkins

[squito]

@vanzin @carsonwang

[vanzin]

Looks good but you have to fix conflicts now.

[SparkQA]

Test build #85274 has finished for PR 20047 at commit 28ee8a9.

• This patch fails to build.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #85271 has finished for PR 20047 at commit 29d184c.

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

[SparkQA]

Test build #85280 has finished for PR 20047 at commit 89e1c6d.

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

[vanzin]

Merging to master.