[V1] Update zmq socket creation in nixl connector

[russellb]

While browsing this code, I noticed this code wasn't using the
make_zmq_socket helper from vllm.utils. Update it to use it.

The benefits are:

• this fixes IPv6 support
• Optimize buffer sizes based on system memory to improve throughput,
  though this part doesn't seem too relevant for how the current sockets
  are used. I still think using the helper is a good best practice.

Fixing IPv6 support also required a fix to how the zmq path string is
constructed to wrap IPv6 addresses in square brackets before adding a
port number to the end.

Signed-off-by: Russell Bryant rbryant@redhat.com

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