p2p/nat: fix concurrent access to autodisc Interface

Concurrent calls to Interface methods on autodisc could return a "not
discovered" error if the discovery did not finish before the call.
autodisc.wait expected the done channel to carry the found Interface
but it was closed instead.

The fix is to use sync.Once for now, which is easier to get right.
And there is a test. Finally.

This will have to change again when we introduce re-discovery.

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