Gracefully handle calls to DefaultPauseDetectorWrapper#retain during shutdown.

[craftmaster2190]

fixes #1883

• Gracefully handle calls to DefaultPauseDetectorWrapper#retain during shutdown
• Use mockito-inline to test static method Runtime#getRuntime()
• Unit test mocked shutdown behavior

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[mp911de]

Thank you for your contribution. That's merged and backported now. I cherry-picked only the actual change as this is quite an edge-case because the client should be shut down before the JVM starts its shutdown process.