[FLINK-15109][runtime] null out restored state ref in InternalTimerServiceImpl after use

[rkhachatryan]

What is the purpose of the change

InternalTimerServiceImpl references restored state after use, taking up resources unnecessarily.
This PR just nullifies its restoredTimersSnapshot field in startTimerService().

Verifying this change

This change is already covered by existing tests, such as InternalTimerServiceImplTest.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

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Automated Checks

Last check on commit 11f8c4e (Fri Dec 06 14:52:04 UTC 2019)

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CI report:

• 11f8c4e : FAILURE Build

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[zhijiangW]

Thanks for this contribution @rkhachatryan !

It is reasonable to release the state reference after used. LGTM!

The travis failure was caused by kafka gelly module which should be irrelevant with this change.