Goal: 

We should get Designate Worker 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 futurist.GreenThreadPoolExecutor is replaced with futurist.ThreadPoolExecutor (or concurrent.futures.ThreadPoolExecutor) in designate/worker/processing.py.
• Verify the Worker service correctly processes DNS record changes and other tasks.
• Verify unit and functional tests related to the Worker service pass.