No functional change as this wasn't actually in use.

This will discard any pending messages immediately when the socket is
closed. The default is to wait forever for any pending messages which
can cause the the daemon to hang on shutdown.

See: http://api.zeromq.org/4-2:zmq-setsockopt

Previously, if data was None, we'd not send a reply. This can lead
to clients hanging when they are expecting a reply.

Symptom was that eswitchd would become unresponsive after some time.
The following error was observed:

  GetVnics failed - [Errno 24] Too many open files: OSError: [Errno 24] Too many open files

This was tracked down to forgetting to call close on IPRoute objects
as per the documentation. See:

- https://docs.pyroute2.org/iproute.html
- svinota/pyroute2#631

The API is defined in:

https://github.com/openstack/neutron-lib/blob/90fa9948fd3b36d121ec5fa0b8004404f6d86b45/neutron_lib/api/definitions/extra_dhcp_opt.py#L32

We were missing the ip_version field. This was causing a bad interaction
with the OVN driver.

See: https://bugs.launchpad.net/neutron/+bug/2081161