Goal: 

We should get Designate Central service to no longer rely on Eventlet's concurrency model, so that its execution environment is standardized on Python's native threading and is easier to debug and maintain.

Acceptance Criteria:

• Verify any futurist.GreenThreadPoolExecutor usages (especially in functional tests interacting with Central) are updated to ThreadPoolExecutor.
• Verify related RPC calls work with using the threading backend.
• Verify the Central service correctly handles zone management, notification processing, and interacts with other services.
• Verify unit and functional tests related to the Central service pass.

Some locations in the Central code where eventlet is used:

• designate/central/service.py
• designate/tests/functional/central/test_service.py