[SPARK-11887] Close PersistenceEngine at the end of PersistenceEngineSuite tests

[JoshRosen]

In PersistenceEngineSuite, we do not call close() on the PersistenceEngine at the end of the test. For the ZooKeeperPersistenceEngine, this causes us to leak a ZooKeeper client, causing the logs of unrelated tests to be periodically spammed with connection error messages from that client:

15/11/20 05:13:35.789 pool-1-thread-1-ScalaTest-running-PersistenceEngineSuite-SendThread(localhost:15741) INFO ClientCnxn: Opening socket connection to server localhost/127.0.0.1:15741. Will not attempt to authenticate using SASL (unknown error)
15/11/20 05:13:35.790 pool-1-thread-1-ScalaTest-running-PersistenceEngineSuite-SendThread(localhost:15741) WARN ClientCnxn: Session 0x15124ff48dd0000 for server null, unexpected error, closing socket connection and attempting reconnect
java.net.ConnectException: Connection refused
at sun.nio.ch.SocketChannelImpl.checkConnect(Native Method)
at sun.nio.ch.SocketChannelImpl.finishConnect(SocketChannelImpl.java:739)
at org.apache.zookeeper.ClientCnxnSocketNIO.doTransport(ClientCnxnSocketNIO.java:350)
at org.apache.zookeeper.ClientCnxn$SendThread.run(ClientCnxn.java:1068)

This patch fixes this by using a finally block.

[JoshRosen]

Reviewable's diff makes it clear that this patch is 99% whitespace changes: https://reviewable.io/reviews/apache/spark/9864

[SparkQA]

Test build #46428 has finished for PR 9864 at commit ea9d063.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[JoshRosen]

/cc @zsxwing for review.

[JoshRosen]

You can look at https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder/46428/artifact/core/target/ to see that this ended up fixing those spurious log messages.

[zsxwing]

Reviewable's diff makes it clear that this patch is 99% whitespace changes: https://reviewable.io/reviews/apache/spark/9864

You can also use https://github.com/apache/spark/pull/9864/files?w=1 to ignore space changes :)

[zsxwing]

LGTM

[JoshRosen]

Woah, cool! I didn't know about that GitHub feature. I'm going to merge this into master and branch-1.6.