[SPARK-4756][SQL] FIX: sessionToActivePool grow infinitely, even as sessions expire

[guowei2]

sessionToActivePool in SparkSQLOperationManager grow infinitely, even as sessions expire.
we should remove the pool value when the session closed, even though sessionToActivePool would not exist in all of sessions.

[AmplabJenkins]

Can one of the admins verify this patch?

[marmbrus]

ok to test

[SparkQA]

Test build #24185 has started for PR 3617 at commit e070998.

• This patch merges cleanly.

[SparkQA]

Test build #24185 has finished for PR 3617 at commit e070998.

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

[AmplabJenkins]

Test FAILed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/24185/
Test FAILed.

[guowei2]

Sorry for that.
i have a mistake about not test with shim12

[SparkQA]

Test build #24204 has started for PR 3617 at commit e9b97b8.

• This patch merges cleanly.

[SparkQA]

Test build #24204 has finished for PR 3617 at commit e9b97b8.

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

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/24204/
Test PASSed.

[marmbrus]

/cc @liancheng

[liancheng]

This is a known issue, and the solution provided in this PR LGTM, thanks!

[marmbrus]

Thanks! Merged to master.