Simple mDNS + DNS-SD impl

Main TODOs:
1. the spec requires the impl to resend the reply once and
   to send gratuitous updates

2. advertise our clubby services as DNS-SD sub-services.

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Tested with:

1. https://play.google.com/store/apps/details?id=com.grokkt.android.bonjour&hl=en
2. https://play.google.com/store/apps/details?id=com.druk.bonjour.browser&hl=en
3. osx `dns-sd`

Works well enough; it doesn't have some glitches due the fact that we
don't retransmit yet and we don't send gratuitous updates.

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