This PR adds a compatibility class that allows a js-libp2p node to find a go-libp2p node (and vice versa) over MDNS.

It's implemented as a separate class so the two differing implementations do not get confused.

I've verified this is working correctly by running a go-ipfs and js-ipfs node with no boostrap nodes (and no other discovery methods) and verifying they find each other.

TODO:

* [ ] Add tests!

Some tips if you want to try this out:

* After you've run `ipfs init`, remember to remove the bootstrap nodes from the config file (`~/.ipfs/config`) of each node before you start up
* Use `ipfs log level mdns debug` for some go-ipfs mdns logs
* You can use the following script (after `npm link`ing this branch) to start a js-ipfs node with no bootstrap nodes and no discovery modules other than MDNS:

```js
const IPFS = require('ipfs')
const MDNS = require('libp2p-mdns')
const TCP = require('libp2p-tcp')

const ipfs = new IPFS({
  repo: '/tmp/ipfs-mdns',
  config: {
    Bootstrap: []
  },
  libp2p: {
    modules: {
      peerDiscovery: [MDNS],
      transport: [TCP]
    }
  }
})

ipfs.on('ready', async () => {
  console.log('ipfs is ready')
  console.log('My Peer ID:', (await ipfs.id()).id)
  setInterval(async () => {
    const peers = await ipfs.swarm.peers()
    console.log(peers.length, 'peers:')
    peers.forEach(p => console.log(p.peer.toB58String()))
  }, 10000)
})
```

License: MIT
Signed-off-by: Alan Shaw <alan.shaw@protocol.ai>

License: MIT
Signed-off-by: Alan Shaw <alan.shaw@protocol.ai>

License: MIT
Signed-off-by: Alan Shaw <alan.shaw@protocol.ai>

Co-Authored-By: alanshaw <alan.shaw@protocol.ai>

License: MIT
Signed-off-by: Alan Shaw <alan@tableflip.io>

License: MIT
Signed-off-by: Alan Shaw <alan@tableflip.io>

License: MIT
Signed-off-by: Alan Shaw <alan@tableflip.io>

License: MIT
Signed-off-by: Alan Shaw <alan@tableflip.io>

License: MIT
Signed-off-by: Alan Shaw <alan@tableflip.io>

License: MIT
Signed-off-by: Alan Shaw <alan@tableflip.io>

License: MIT
Signed-off-by: Alan Shaw <alan@tableflip.io>