Listen on the given host/port and fire onlink(Link).

    Returns a server object that contains the listening sockets

async def listen(host, port, onlink, ssl=None):
    '''
    Listen on the given host/port and fire onlink(Link).

    Returns a server object that contains the listening sockets
    '''
    async def onconn(reader, writer):
        link = await Link.anit(reader, writer)
        link.schedCoro(onlink(link))

    server = await asyncio.start_server(onconn, host=host, port=port, ssl=ssl)
    return server