properly shutdown zmq socket

[AionJayT]

Description

Please include a brief summary of the change that this pull request proposes. Include any relevant motivation and context. List any dependencies required for this change.

• Zmq socket doesn't shutdown properly. It causes the client side native zmq lib has a chance to throw errors and crash the application when the kernel is shutting down.

Fixes Issue # .

Type of change

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• Bug fix.
• New feature.
• Breaking change (a fix or feature that causes existing functionality to not work as expected).
• Requires documentation update.

Testing

Please describe the tests you used to validate this pull request. Provide any relevant details for test configurations as well as any instructions to reproduce these results.

• use javaAPI client API test suite to verify the issues.

Verification

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• I have self-reviewed my own code and conformed to the style guidelines of this project.
• New and existing tests pass locally with my changes.
• I have added tests for my fix or feature.
• I have made appropriate changes to the corresponding documentation.
• My code generates no new warnings.
• Any dependent changes have been made.

[ali-sharif]

Can verify that graceful kernel shutdowns won't cause client to ungracefully shutdown. On the client-side, still the issue that if the kernel is not shutdown gracefully, cleint causes consuming application to crash (see aionnetwork/aion_api#26)