Goal: 

• We should get Designate Mdns 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 Mdns service uses native Python threading instead of eventlet green threads for handling concurrent DNS queries (TCP and UDP).
• Verify the mDNS service correctly handles DNS queries, NOTIFY messages, and AXFR requests.
• Verify unit and functional tests related to Mdns service pass / are adapted.

  Some locations in the mDNS code where eventlet may be used:

• designate/service.py (DNSService class - TCP/UDP handlers)
• designate/mdns/service.py
• designate/tests/unit/mdns/test_service.py