Change DubboShutdownHook to singleton and process level

[Chenjp]

When we register DubboShutdownHook, actions-on-exit are expected. So we should not disable the Hook.

What is the purpose of the change

Change DubboShutdownHook to singleton, and make it as process level Hook. Fix #14429.

Brief changelog

As declared, ShutdownHook should perform process-exit ops, familiar with dubbo engine. Move DubboShutdownHook#addShutdownHook back to DubboBootstrap.

Update DubboShutdownHookTest to add more internal check and improve code coverage.

Verifying this change

Checklist

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