Goal:

We should get Designate Producer 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 the Producer service entry point is migrated from designate/cmd/eventlet/producer.py to designate/cmd/threading/producer.py
• Verify RPC initialization is handled correctly in the threading-based service lifecycle (RPC initialized before heartbeat emitter creation).
• Verify the Producer service correctly executes periodic tasks (zone serial increments, delayed notifications, etc.).
• Verify heartbeat emissions to the Central service work properly.
• Verify unit and functional tests related to the Producer service pass.

  Locations where eventlet is removed:

  - designate/cmd/eventlet/producer.py → designate/cmd/threading/producer.py
  - designate/service.py (RPC initialization timing)
  - All service entry points (explicit RPC init is added before heartbeat emitter creation)