fix the instance name used for the zeroconf server

[marten-seemann]

Fixes #1207. Requesting a review from @mxinden, since this is more of a spec-compliance issue here than a coding issue.

The effect would be that the host name we use is not \018\ *\198*\229\139\238e\188..\002K?\172#\'\000\249LY\248<\177\131\ \246<\2081\154\009._p2p._udp.local., but Qmxxx._p2p._udp.local.

[mxinden]

The peer name would need to be base-32 encoded, right?

See spec:

peer-name is the case-insensitive unique identifier of the peer, and is less than 64 characters. It is normally the base-32 encoding of the peer's ID.

https://github.com/libp2p/specs/blob/master/discovery/mdns.md#definitions

Logic that encodes a peer id in rust-libp2p, making sure it does not exceed the 63 byte max label length:

https://github.com/libp2p/rust-libp2p/blob/c13f03354b8ec179845df825afbddb6665decaba/protocols/mdns/src/dns.rs#L283-L297

In practice my peer with the peer ID 12D3KooWM72jxc25yR4sHYA25wFUP3k5STpwpzYvkGTYwsTgGxnV emits the following MDNS packet:

Frame 135: 522 bytes on wire (4176 bits), 522 bytes captured (4176 bits) on interface wlp3s0, id 0
Ethernet II, Src: IntelCor_8c:3e:b2 (84:fd:d1:8c:3e:b2), Dst: IPv4mcast_fb (01:00:5e:00:00:fb)
Internet Protocol Version 4, Src: 192.168.2.107, Dst: 224.0.0.251
User Datagram Protocol, Src Port: 34988, Dst Port: 5353
Multicast Domain Name System (response)
    Transaction ID: 0xbb4d
    Flags: 0x8400 Standard query response, No error
    Questions: 0
    Answer RRs: 1
    Authority RRs: 0
    Additional RRs: 2
    Answers
        _p2p._udp.local: type PTR, class IN, 009jj0820t9ccv5nw8hw82umphjc6dxxnqqt760hkfwcrmnmgc53vd0vzyu49._p2p._udp.local
            Name: _p2p._udp.local
            Type: PTR (domain name PoinTeR) (12)
            .000 0000 0000 0001 = Class: IN (0x0001)
            0... .... .... .... = Cache flush: False
            Time to live: 360 (6 minutes)
            Data length: 79
            Domain Name: 009jj0820t9ccv5nw8hw82umphjc6dxxnqqt760hkfwcrmnmgc53vd0vzyu49._p2p._udp.local
    Additional records
    [Request In: 134]
    [Time: 0.000419826 seconds]