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That's what the Swarm does and that's what the function's documentation says it
does.

The swarm does this as well and most of our services will fail if we don't have
this.

mocknet: use peer ID in peer address

RandLocalTCPAddress is mostly useful when we *actually* want to listen on a real
address. Unfortunately, when running a bunch of tests, we can actually run
out.

With this change, a collision means we have a duplicate peer ID so yeah...

fixes #473

autorelay

also check that we don't initially advertise any.

macosx seems to fail intermittently, and that race is the likely culprit.