Implement peer discovery on LAN using mDNS (via `zeroconf`).
A Peer ID is generated (via `libP2P`) per instance, and announced on backend startup.
Meanwhile, other peers are discovered on LAN. A signal is send on peer found and another
on peer lost.
There a bridge methods to find already discovered peers, and to get infos on serverless
(our own ID, adverised service name, and number of peers found).
`Zeroconf` runs in background in its own thread.
rel 479
