Should return a new object every time

[RangerMauve]

There's a bug in discovery-swarm-stream when you try to use it with dat-swarm-defaults multiple times.

When UTP is enabled, it sets opts.dns.socket to a value. This socket will then get globally reused across all future calls of dat-swarm-defaults because opts.dns references a global object.

The symptoms are that you can't close the discovery swarm since the dns-discovery code is calling .close() on this global socket which is already closed from the previous run.

Of course we should probably fix this inside discovery-swarm, but I think it'd be better to avoid this issue happening in any other modules where developers might not be aware of the potential error.