feat!: frontend should handle peer discovery

[EvanHahn]

See also: digidem/comapeo-mobile#245.

I recommend reviewing this one commit at a time.

Our peer discovery system is unreliable on some networks. We'd like to replace it with a native local service discovery system.

To do this, the frontend needs to tell the backend about peers it finds. In other words, we need to "invert" peer discovery.

Here's pseudocode for what the frontend needs to do:

const { name, port } = await mapeoApi.startLocalPeerDiscoveryServer()
zeroconf.publishService({ name, port })
zeroconf.on('resolved', (peer) => {
  mapeoApi.connectPeer(peer)
})

This implements that by removing our mDNS system and replacing it with these public APIs.

Closes #474.

[achou11]

changes seem good to me!

[achou11]

nit: do you think it will it ever be useful to provide a custom name? can consider a constructor opt for this if so

[EvanHahn]

I think it might be useful in future but not at present, so I'm going to skip it for now.