[ZEPPELIN-4390]. ExecutorService is not properly shutdown

[zjffdu]

What is this PR for?

ExecutorService is not properly shutdown due to we didn't use the correct api. We should use shutdownNow instead of shutdown. See https://stackoverflow.com/questions/11520189/difference-between-shutdown-and-shutdownnow-of-executor-service.

The effect of this issue will cause thread resource leakage.

What type of PR is it?

[Improvement]

Todos

• - Task

What is the Jira issue?

• https://issues.apache.org/jira/browse/ZEPPELIN-4390

How should this be tested?

• CI pass

Screenshots (if appropriate)

Questions:

• Does the licenses files need update? No
• Is there breaking changes for older versions? No
• Does this needs documentation? No

[zjffdu]

Will merge if no more comments