Race condition in DHCP agent cause deletion of the dhcp port as stale port

It seems that when there are 2 subnets using different segments created on same network, it can cause race condition and one of the green threads will expect tap port to be there but other will treat it as "stale" device and will remove it from namespace.

It causes failures in the neutron.tests.fullstack.test_multisegs.TestMultiSegs.test_multi_segs_network pretty often.

Stacktrace example:

ft1.1: neutron.tests.fullstack.test_multisegs.TestMultiSegs.test_multi_segs_network(Open vSwitch Agent)testtools.testresult.real._StringException: Traceback (most recent call last):
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/common/utils.py", line 743, in wait_until_true
    eventlet.sleep(sleep)
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-fullstack-gate/lib/python3.10/site-packages/eventlet/greenthread.py", line 36, in sleep
    hub.switch()
  File "/home/zuul/src/opendev.org/openstack/neutron/.tox/dsvm-fullstack-gate/lib/python3.10/site-packages/eventlet/hubs/hub.py", line 313, in switch
    return self.greenlet.switch()
eventlet.timeout.Timeout: 60 seconds

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/base.py", line 182, in func
    return f(self, *args, **kwargs)
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/fullstack/test_multisegs.py", line 146, in test_multi_segs_network
    self.vm1 = self._spawn_vm(neutron_port=self.port1)
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/fullstack/test_multisegs.py", line 65, in _spawn_vm
    vm.block_until_dhcp_config_done()
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/tests/fullstack/resources/machine.py", line 217, in block_until_dhcp_config_done
    utils.wait_until_true(
  File "/home/zuul/src/opendev.org/openstack/neutron/neutron/common/utils.py", line 747, in wait_until_true
    raise exception
neutron.tests.common.machine_fixtures.FakeMachineException: Address 10.0.11.195/24 or gateway 10.0.11.1 not configured properly on port portbc2128

Failure example: https://750440148c983d142b4c-dd0601cbd7f0aa15c09a82437ec7e47b.ssl.cf5.rackcdn.com/872396/2/gate/neutron-fullstack-with-uwsgi/c5e45ed/testr_results.html