Fix sliding window node failed to restore from check/save point

[daugraph]

This close #256

What is the purpose of the change

(For example: This pull request introduces Flink Processor.)

Brief change log

(for example:)

• Introduces Flink Processor
• Adds examples and documentations of Flink Processor

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API: (yes / no)

Documentation

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

[CLAassistant]

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